When I got my Series 2a, it was a hardtop with a roof rack and ladder, the plate at the back had been moved to the right so as not to be obscured by the ladder..
It is I think fair to say then that the plate was proberly made to go on the left from new.
My front one is in the middle, so I can't help you there.
If you look at classic van's that might give you a better indication.