I can't recall off hand if there were any differences in markings: just the better 'ole. If you are looking at a pile of them, the difference is very obvious. And there were only the two sizes, so you don't have 47 more surprises coming. Also, they were dated, so look for a wartime date: Mark III production was just starting when the War started.
I once ended up with an early bayonet myself while looking for a late one. It's just about new, so I haven't got rid of it. Need another Ross, need another RossneedanotherRossNEED...
Oh, no! A relapse!
And the pension cheques aren't in yet!
Good luck in your search, friend.
NEEDANOTH......... AAAAAAAAAArrrrrrrrrrgh!