The South African "RxMx" system of nomenclature is exactly the same as the British Commonwealth "LxAx" system, where "x" represents digits from 1 to wherever.
IN the Ammo field, each calibre had an "L" designator, and these went upwards as variations occured in case design, bullet design, powder changes, etc. Completely changed cartridges were given a new , different "L" designator ( such as L5 for (7,62) tracer, etc, and the "A" designator handled all the Minor changes.
THis was also used in Weapons designs, the "A" being the modifications in time and series ( Just like the US "A" designation).
South Africa originally used the British "L" system, then with increasing Independance and expulsion from the Commonwealth for its Apartheid program, adopted the very similar "R x M x" Notation ( R being "Republic" ( of South Africa) and "M" being Modification...Note that the initials are the same in English and Afrikaans, the Dutch origin language of many in South Africa, and an official Language up till the overthrow of the White Nationalist Gov't.
I concur with the statement that Jane's is often seriously wrong in a lot of matters regarding details of Ammo etc...it is just too big a publication system....when it was only about Naval vessels, it was fine...now they try to cover every man and his dog ( sorry, Gun) and the results are not encouraging.
BTW, even small differances such as the type of neck sealant etc, can affect both the "LxAx" system, as it could affect the "RxMx" system. Different suppliers of British ammo also got different "A" numbers for their ammo ( eg, Hirtenberger 9mm, Kynoch contracts pre 1970, etc.
Regards,
Doc AV
AV Ballistics