The early LSWR/SR suburban stock ('3', later augmented to '4-SUB') of the 1920s motor (end) coaches were fitted with panels/frames to only old a single, letter stencil - and that is the way they stayed until withdrawal. From the 1930s the SR moved to a system that displayed two numeric stencils (for 2 digit numbers), initially on main line stock, and later the Bulleid replacement suburban stock (though only one stencil of each digit was supplied, precluding codes using the same digit twice until roller blinds came along). None of the old stock was converted, so for many years the instructions provided for both systems.
The LMS 1924 code was inherited from the LNWR, and I believe was the same as one used on the NER, and is also the same as that in the GWR 1939 code book (listed by
railway codes website)
So was this a code standardised by the RCH (or ...?) for all railways?
But it not the same as any I've seen for 'contemporary' (pre WW2) use in other fields (armed services, PO telegraphs/telephones, etc.); nor that listed by railway codes as a BR 1950 version.
So is anyone able to give any background as where it originated? And what drove the change between 1939 and 1950 (and when was this? was there an itermediate stage?)?
How, when and to what extent were these codes actually used, until the relatively recent adoption of the international NATO/ICAO standard code/alphabet?