ZTSC, J ZSP etc
The last letter had nothing to do with it being the A or B side, it is to do cataloguing and press run for that song, the highest I've seen was F. However, I think the precise reasoning is a bit complicanted than A= 1st B= 2nd etc as Giggi & Charmains - Brazil is 1A and the other side 'poor unfortunate me' is 1D. Obviously, there are lots of 1A's and 1B's as most records were only pressed up once, or twice maximum, and so that's why it seems to be related to 'A' & 'B' sides.