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History of Communications-Electronics in the United States Navy, Captain Linwood S. Howeth, USN (Retired), 1963, page 575:

Appendix  H.  The  U.  S.  Navy  Standard  Drawing  Number  System

This was a part of a general coordination system drawn up by the Navy in 1914-15. In this system, drawings, reports, photographs, lists, specifications, etc., were filed according to origin, class, size, serial number, and alteration.
    All of these characteristics were expressed in the identifying number which appeared in the lower right-hand corner of the sheet, be it drawing, typed statement, photo, etc. Typical of this would be a drawing numbered "RW  46A  114  B." Analyzing this:
  1. "R" identified the drawing as being of the radio classification.
  2. "W" indicated that yard which originated the drawing - - in this case, the Washington Navy Yard.
  3. "46" was the class applying to the subject treated. In the example, the drawing is of a radio receiver, to which class 46 has been assigned.
  4. "A" indicated the size of the sheet. There were four sizes. Size A, as here used, was identical with the Navy typewriter paper size - - 8"X10½", slightly smaller than commercial 8½"X11" size.
  5. "114" was the serial number. This meant that this drawing was No. 114 in Class 46 as prepared by Washington Yard. The preceding numbers from No. 1 on might have been all reports, or all photos, or--as probable--a mixture; as long as they referred to receivers, they received the "next serial number" in their turn.
  6. "B" was the alteration letter. It meant that there had been an original drawing with the same serial number but no final letter; the first alteration received the letter "A," and this present alteration, the second, received the letter "B." Generalities of the system:

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