I’m still figuring out details about this bike, do I understand correctly that there were different barrel designs?
Yeah, some models were plain iron bores and some had chrome bores. Haven't dug deep enough yet to know which is which. I believe square or round refers to the fins. If that's correct, this one's a square barrel.
Found an interesting discussion on Britbike.com re: square barrels vs round
https://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=413963