Interesting indeed!!! Looks a bit like an early attempt before the factory "High Boy" 1966 variation...
I've just recently re-read "Mann of his time" by Ed Youngblood, seems that the AMA had to loosen the belt regarding frames after the G50 Matchless homologation problems of 63/64 (they allowed Mann to use the G50 frame in TT but not in road racing!!!). After that, they all started variations on the std frames, Belland, Lawwill, Branch and the factory soon followed!!!
So I would personnally date it sometime after 64.
See you've got the nice radially finned race rear drum on that as well... Keep on finding rare bits!!!
All the best from Patrick