Officially, the international Morse code distress signal is: ...---... i.e. three-dots/three-dashes/three-dots all run together, and not really the three separate letters "SOS". But when talking about the distress signal informally, it's usually broken into three even parts, which becomes "SOS" in International Morse code.
 
The Electrician, May 5, 1905, pages 94-95:

GERMAN  REGULATIONS  FOR  THE  CONTROL  OF  SPARK  TELEGRAPHY.
    A code of regulations issued by the German Government for the control of wireless or "spark" telegraphy came into force last month.
    The first regulation defines a coast station as one erected on the mainland or an island, permanently anchored ships being also included under the definition "coast stations" and not under "ship stations." The International Service Regulations, as revised in London in 1903, and the German Government Telegraph Regulations with regard to submarine telegrams are to apply to wireless telegraphy across the sea unless modified by any of the present special regulations. The interchange of wireless telegrams with the Government telegraphic network is to be in accordance with the regulations for submarine telegrams, and the messages may be written in any of the languages allowed under the International Service Regulations above referred to.
    Regulation 2 defines the scope of the present regulations. They apply to wireless telegraphic traffic to and from the German public coast stations, as defined above.
    Stations on lightships as a rule only communicate with a given coast station. They may receive private as well as official telegrams, whether incoming or outgoing, addressed to the crew of the ship, and may receive messages or signals from ships not sent by wireless telegraphy, for further transmission. They may not, however, communicate with ships by wireless telegraphy except in cases of emergency.
    The present regulations have also to be followed by coast stations which are not public coast stations--i.e., those given under 5b,--and also, as far as applicable, by ships sailing under the German flag when communicating with one another.
    Regulation 3 prescribes that every public coast station of the German Admiralty is to be ready to work at any time of the day or night, except in so far as its operation has been stopped owing to exigencies of service for manoeuvres, or for testing purposes.
    Public coast stations are to work with a wave-length of 365 metres; their range for communication with a station similarly equipped is to be 200 km.; and their range to a ship with a 30-metre mast 120 km., under normal weather conditions.
    Regulation 4 prescribes that ordinary Morse signals are to be used, together with the following additional signals :-
    ------, "Cease sending" signal (Ruhezeichen). This may only be given by public coast stations.
    ...---..., Distress signal (Notzeichen). This is to be repeated by a ship in distress until all other stations have stopped working.
    ...---., Quest signal (Suchzeichen). This may be repeated by ships on the high sea, the signal to be followed by the name of the ship. It is to be replied to by the word "hier" (here), followed by the name of the replier.
    Wireless telegrams are to be dealt with in the following order (1) State telegrams, (2) service telegrams, (3) urgent private telegrams, (4) ordinary private telegrams, but messages from ships in distress have precedence over all the above. If the distress signal is not followed by any code name, every station that receives the signal is to signify its receipt of it.
    It is the duty of public coast stations to exchange wireless telegraph messages with ship stations within their maximum range. Preference must be given to the ship which will first leave the zone of intelligible communication, account being taken of its position, equipment and speed. A ship may only communicate with the coast station nearest to it except in the case of its being unable to communicate intelligibly with this station, owing to defective working, &c. A ship must not call up a coast station until it is within the certain range of the station. This range is to be taken at three-quarters of the highest range over which the coast station can communicate with the ship. Every ship must employ that one of its wave-lengths which is nearest to the wave-length of the coast station. Before calling up, any station, whether on a ship or on shore, must ascertain by means of its listening apparatus (or, if it has none, its receiving apparatus set to its condition of greatest sensitiveness), whether any other messages are being transmitted, in which case it must wait until these have finished. All stations must work their transmitting apparatus at the smallest effective intensity.
    On the receipt of the "Cease sending" signal from a public coast station, a ship must immediately cease transmitting and must only resume when it is instructed to do so.
    If the call from a ship is not answered by the coast station, the ship may repeat it three times with an interval of not less than five minutes between each time; if the fourth call is not replied to, the ship may call the station again after an interval of one hour.
    Addresses on telegrams to ships at sea must bear the name of the addressee, the name of the coast station through which the telegram is to be transmitted, the name of the ship or its official number and the nationality of the ship. Telegrams received from ships at sea will be headed with the name of the coast station as the transmitting office, and after this the name of the ship is added.
    Wireless telegrams intended for ships are only sent if the ship calls up the coast station in passing, and only so long as the ship is within range of the coast station.
    If a ship, to which a wireless message has been directed, does not report itself to the coast station within the period indicated by the sender or within 29 days, this fact is notified to the sender. At the sender's request, however, the telegram may then be held at the coast station for transmission to the ship for a further period of 30 days.
    In addition to the ordinary charge, a supplementary charge of 80pf. is made for each wireless telegram, this charge being paid by the sender if the telegram is sent to a ship, and by the recipient if it is sent from a ship. In cases where the charges have already been fixed, however, such charges will remain in force for the present.
    A ship, when calling a shore station, is to begin with the call signal, followed by three repetitions of the code name of the coast station, then the letter v, and its own code name (or, if does not possess one, the name of the ship). The coast station replies by giving successively the call signal, the code name of the ship, the letter v, and its own code name. This is followed by -.-, should the coast station be ready to receive; or by the signal to wait (.-...) followed by the number of minutes of the probable delay, and, if this is in excess of 10 minutes, the reason for such delay. If the coast station has given the signal to wait, the ship must await a further call.
    The ship is to commence its telegram by means of one of the following instructions :--s, ss, a, d, indicating respectively a Government telegram, free Government telegram, service telegram, urgent private telegram. Then is to follow the destination, v or de, the name of the ship, the number of the telegram (if the telegram is destined for abroad), the number of words, and the time of handing in. The latter is to be indicated by three figures : day of month, hour and minutes. After this preamble, the supplementary instructions, the address, the text and the signature of the telegram are successively transmitted.

    Regulation 5 gives a list of the code names of the German coast stations and merchant vessels :--
a. Public Coast Stations.
  1.  Rixhöft.k r x    |    5.  Heligoland.k h g
  2.  Arcona.k a r    |    6.  Cuxhaven.k c x
  3.  Marienleuchte.k m r    |    7.  Borkum lighthouse.k b m
  4.  Bülk.k b k    |    8.  Borkumriff lightship.f b r
b.  Coast Stations for Limited Public Traffic.
  1.  Bremerhaven Lloydhalle.k b h
  2.  Weser Lightship.f w f
  3.  Elbe I. Lightship.f e f
c.  Merchant Steamers.
  1.  " Kaiser Wilhelm der Grosse "  North German Lloydd k w
  2.  " Kronprinz Wilhelm "dittod k p
  3.  " Kaiser Wilhelm II. "dittod k m
  4.  " Deutschland "Hamburg Americand d l
  5.  " Moltke "dittod d m
  6.  " Blücher "dittod d b
  7.  " König Albert "dittod k a
  8.  " Meteor "dittod m r
  9.  " Cap Ortegal "Hamburg S. American  d c o
10.  " Cap Blanco "dittod c b
11.  " Prinz Adalbert "Kiel-Korsörd p a
12.  " Prinz Sigismund "dittod p s
13.  " Prinz Waldemar "dittod p w

    Code names are fixed by the department which gives permission for the installation of wireless telegraph stations.