There are some technical differences between the early ones and later ones.
Early ones have a brass capstan and occasionally a CMC-4 instead of a nickel capstan and CMC-10.
The brass capstan is actually a bit better than the nickel one because the metal holds on to very slight moisture (less static) and grabs tapes slightly better.
They will also have a WME289 and WME290 circuit boards for volume and tone.
The later ones have Sound Sales mods like the PML board which unifies volume and tone on one board, and
also have SMS2 motor boards, which is where the SM reference comes from.
Sound Sales units also have different equalization and possibly even different takes for work master tapes than the original BBS ones.
I believe these were the 'Mellotronics Ltd' tape work masters and not Streetly's work masters.
For example you don't get that loud high D note at the beginning of King Crimson's 'Trio' when played from a Mellotronics or Sound Sales tape set. You can only get that 'feature' from an original BBS set (or a copy of those exact takes in the masters). The flute is also more breathy.
Conversely on the other hand, the strings sound more brash and upfront and a bit less warm in the Sound Sales tapes.
You can hear that on Strange Advance' 'Worlds Away'. There are differences in some other sounds too.
Sound variance also depends on studio trickery as well, and whether a machine was customized.
Of course, being able to detect these differences depends on whether your machine is set up properly - not just the pinch rollers, pressure pads, but also the speed and stability controls on the SMS circuit boards (which are almost never talked about - or put back into proper adjustment). If everything isn't set up right, then these subtleties are completely lost and you'll never hear them or even know they exist.
As far as numbers go, I think there's an M400A, M400B, M400C, M400D, and finally the M400SM.
I don't know the exact differences between them, but they will have combined variations on the above mentioned details.
Others here might be able to elucidate better than me.