Until the Department of Commerce started licencing U.S. radio stations in late 1912, amateur radio transmitters were unregulated, and could use any wavelength and callsign desired. In this period before the beginning of regulation, Modern Electrics began publishing in 1909 an annual "Blue Book" list of stations, to help amateurs organize. As noted below, as part of the registration process the magazine assigned call letters to amateurs who hadn't already chosen ones for themselves. Although this list doesn't indicate which of the listed calls were self-assigned, and which were created by the magazine, it is likely most of these calls were magazine-issued, since according to other lists of this period most calls amateurs assigned themselves were only two letters.

Many of the calls below appear to have been created by starting with two letters based on the person's name, followed by a third letter of "M", possibly signifying "Modern Electrics". And one of these calls is of particular interest -- the eighth person on the list, Earl C. Hawkins of Minneapolis, Minnesota, is listed as "H.A.M.", apparently created by combining the first two letters of his last name with a closing "M". The obvious question is whether this was done on purpose, and an early example of an amateur being referred to as "a ham", or was it just a coincidence. (There is a often repeated, but purely fanciful, story which claims that around this same time a Harvard University station had the callsign of "HAM", but there is no evidence that such a station ever really existed.
 
Modern Electrics, May, 1909, page 55:

Wireless  Association  of  America
_____________________

Wireless  Registry

      This department has been started with the idea to bring the wireless amateur in closer touch with commercial land and ship stations. Each month a list of new members will be printed here and once each year an official BLUE BOOK will be issued by MODERN ELECTRICS giving a list of all the members who registered during the year. Each member will receive the Official Blue Book free of charge. The Blue Book will also contain a complete list of commercial and government stations, their call letters, wave length, etc.
      To register a station requires: Total length of aerial (from top to spark balls), spark length, call letter, (if none is in existence M. E. will oppoint one) name and address of owner.
      Fee for Registery (including one Blue Book) 25 cents.

  
  NAME  AND  ADDRESS  OF  OWNER.  
  
CALL
   LETTER   
  
APPROXIMATE
  WAVE  LENGTH  
IN  METERS
  SPARK  LENGTH  
OF  INDUCTION
COIL.
Ralph Damon, Whitman, Mass.,R.D.M.175   2 ins.
George and Fred Taylor, Susanville, Cal.,G.F.T.  61   1   "
Karl T. Spark, S. Butte, Mont.,K.S.M.400   3   "
J. A. Marriott, Seattle, Wash.,J.A.M.120   1½   "
Thos. Appleby, W. Phila., Pa.,H.N.M.360   1½   "
H. M. Atwater, Kentfield, Cal.,K.N.M. 1   "
J. Winne, Jr., New York City,J.W.M.150   2   "
Earl C. Hawkins, Minneapolis, Minn.,H.A.M.  62   1½   "
Charles J. Meyers, Albany, N. Y.,C.J.M.175   1   "
Harry Butts, Mazomaine, Wis.,H.B.M.170   1¼   "
S. W. Case, Marcellus, N. Y.,S.W.C.335   2 K W
Geo. H. Nixon, Hackettstown, N. J.,G.H.N.365   1 ins.
Walter E. Haaren, New York City,W.E.M.135   2½   "
Alvah Ritterhouse, Philadelphia, Pa.,F.R.M.  80   3   "
G. S. Vernam, Brooklyn, N. Y.,G.S.W.245   1½   "
Meade N. Powell, St. Louis, Mo.,M.P.M.135   2   "
Rochester Wireless Tele. Co., Rochester, N. Y.,H.125   2   "
Allen Lee Whitman, Cambridge, Mass.,A.L.W.110   ¼   "
Max Wells, Riverside, Cal.,M.W.X.  85   3   "
*Frank McGram, Jersey City, N. J.,F.M.X.  64
Edwin R. Willard, Chicago, Ill.,E.R.W.110   ¼ K W
J. J. Rodrick, Veary, Boston,R.V.M.  35   2½ ins.
Geo. Burrell, Avon, Ohio,G.B.A.125   ½   "
J. C. Randall, Albany, N. Y.,S-4.130   4   "
Chas. F. Spinney, Sanford, Me.,C.E.S.135   2   "
*R. S. Burtt, N. Paterson, N. J.,R.B.M.210
Earle L. M. Coolidge, Everett, Mass.,E.M.C.  70   1½   "
F. Wommer, Minneapolis, Minn.,F.W.M.  74   1½   "
Ray Newby, San Jose, Cal.,E.Z.M.235   3   "
E. D. Porter, Lewistown, Mont.,P.O.M.950   2 K W
Paul J. Hackett, Walla Walla, Wash.,P.J.H.165   3 K W
Thos. J. P. Shannon, Los Angeles, Cal.,S.I.M.160   2 K W
*B. F. Rittenhouse, N. Woodbury, N. J.,A.R.M.105
A. G. Raycraft, Manhattan, Nev.,A.R.M.260   12   "
A. G. Raycraft, Tonopah, Nev.,N.H.M.260   12   "
A. P. Morgan, U. Montclair, N. J.,M.M.U.350   3 K W
E. H. Guilford, Winchester, Mass.,H.H.G.350   3 K W
Harry Brandt, Govans, Md.,S.S.S.  52   1 in.
C. A. Stone, New Haven, Conn.,G.K.S.  35   1   "
Ralph T. Morse, Indianapolis, Ind.,R.T.M.145   2   "
Mervyn Sullivan, San Francisco, Cal.,M.X.M.110   1   "
Kendrick Noble, Yonkers, N. Y.,K.N.Y.  85   ½   "
J. L. Douglas, E. Orange, N. J.,J.L.D.210   1   "
John Christianson, Chicago, Ill.,J.C.C.  45   2   "
Branson M. DeCou, Blairstown, N. J.,B.M.D.180   6   "
A. Inderbitzen, New York City,A-1.200   2   "
Malcolm H. Smith, Gloucester, Mass.,M.S.M.170   ¼ K W
E. L. Showers, Los Angeles, Cal.,E.L.S.270   2 ins.
Wm. Kuhn, Los Angeles, Cal.,W.E.K.270   2   "
F. W. Harris, Renton, Wash.,3-B.235   5½   "
___________
      *Receiving stations only.

      Above stations will be found in the Official Blue Book just issued. Subsequent stations will be registered in next year's blue book.