Indeed a very good question, Phil, one to which I do not know the answer!
Like you I have both crimped and uncrimped ball rounds, but I will go and check what dates/manufacturers they are and post details in a while. All ball rounds should have been ring crimped in British service, so I do not know the expalnation for the uncrimped ones.
The extra stab crimping on the Incendiary B Mark Iz is because the B.Iz was prone to feed prematures in the breech and it was an extra precaution, especially as the open breech was by the gunners ear! Many tank gunners took to hanging a sheet of sacking btween the Besa and their head, although I am not sure how much good that would have done! The BIz was only in service for a short time and was quickly replaced by the BIIz.
I will have a look also to see if there are any references to this in the SAC Minutes/O.B. Procs.
HB857 - Yes, Kynoch ammunition was sent to Chima in WW2, but it was all normal production BESA ammo. Picture shows crate label for Kynoch K44 GIIz tracer.