Can't tell you a whole heck of lot, friend.
Major Peter Labbett wrote a whole series of articles which were published in GUNS REVIEW Magazine (Ravenhill Publishing, London) entitled "The Banded .303s". They were published back in the mid-1960s and I believe they were later collected into a small book. That is the only definitive source I know of. The articles were written in Major Labbett's characteristic style: factual information, closely packed, with just a touch of humour in the odd place.
HOW could you sell your .303s? That's like selling your FRIENDS!!!!
Nice to know there are still a few of these around in nice condition. The Primers have had 74 years to get dodgy, but that is still enough to make 20 collectors smile for a day each.
Hopefully TonyE might have something on these; I believe he has most of Major Labbett's notes.
Nice find!