I'm not an S1A or B specialist, so these are just guesses, I don't have to robinshaw book near me, but that book is a good indication of production changes.
I've seen three S1s in the flesh (#2, #40 and #70) (but none of them are A or Bs) so I have a good background in those, but even in 40 cars there were some differences in the way they were built.
S1: fixed windows, bonded in chassis, lancia rear lights
Guesses below :
S1A: removable windows, lancia lights, bonded in chassis
S1B: removable windows, lucas lights, (removable chassis?)
Serge