The last word on ID will be from the Royal Enfield Owners Club in England who have the surviving records. Go to www.royalenfields.com
and look for chairman Graham Scarth's blurb on the right sidebar.
There is a hasty copy of the engine factory records in circulation. It indicates the engine was built in early 1959, intended for a Super Meteor, altho experience is many of them ended up in Indian Trailblazers. The gearbox number that counts is the number on the front of the crankcase, probably on the bike's right side, or your left when facing the numbers. The gearbox number is not on the covers, but on the top of the main case, near the joint with the covers. On the bike's right side, small, and often covered by grime. Enfield bought gearboxes from Albion and, after assembling with an engine, its' number was copied onto the crankcase. In your engine's case, likely H followed by 2 letters and 3 numerals.