I think AB is pan-head pozi , AC is countersunk. pozi The WA, WB etc I think refer to the washer form letter (eg linky). SH is hex head screw (machine screw, no smooth part on the shank). The SA and SE (SE might be pozi head). I think NY is the 'standard' nylock . These are actually Austin-Morris standards that were imposed across the whole empire, which is why you'll see on the earlier LRs they have a different numbering system (for sometimes the same fastener) from before the black hole that was British Leyland got them. There did used to be a standards guide, I can't believe all copies have been annihilated.