I'm currently doing some research on the now long retired Special Atomic Demolition Munition (SADM).

From my research, I know the SADM had two different model (?) numbers: XM129 and XM159, which I believe is the B54 Mod 1 and B54 Mod 2 devices respectively. Further, each model had two suffixes: E1 and E2, as in XM129E1 etc for a total of 4 versions.

I know in other types of equipment, "A" (like M16A4 etc) is used for successive improvements to a piece of equipment. Does anyone know what "E" means? I assume in this case E1 and E2 means different yield variants of the weapon, but I'm guessing there.

Does anyone know what it takes for a piece of equipment to from the "X" prefix? I assume it means an experimental or not properly certified piece of equipment, but from my research it looks like the SADM retained the X prefix for its entire service length (1963 to 1988), so it seems odd to retain it.

I need an actual source I can cite for what each of these suffixes means, by the way. By that I mean a source I can cite in a paper. Sorry, but "someone on Reddit said so" won't actually suffice.

Cheers.