Don't forget the exhausts, clocks, airbox, seat, plus mudguard transition on the later ones .....probably some more things if my recall was tip top...Lefthand switch gear pass button (or did that first appear on the A) ..then the introduction of the second carb difference ( 522 01) carb, with the casting used on later models but with the inspection port un drilled.
For me its what makes the 521 B so interesting, so much change for relatively few produced.
The model prefixes are of-course 521 and 522. VIN indexes generally start at X00100, the first 100 of any model being the preproduction prototypes. The first digit of the VIN index is typically used to identify year or geographic region.
I've always been curious to figure out how many 350 'B's came to Europe and in what VIN rage groups as generally all UK bikes have VINs in the 000100-002500. Some of the German ones were 1xxxxx but they also got some of the 0xxxxx. I've not seen a UK 1xxxxx.
One day I'll have a stab at collating a profile based on info that available, but right now its a struggle just to find a few minutes to get in the shed and tinker!
