For the most part I don't think 1950 Longbranch rifles came with 6 groove barrel's. All the ones I've seen ( looked at 4 most recently) came with 5 groove barrel's with the odd exception. . Most of the 1950 LB rifles.....under the rear hand guard will be '50 stamped as well as proof marked, CA stamped and have a "I" stamp....I believe this refers to the Mk1 five groove barrel.The 6 groove barrels are CMK 4 and DOM stamped (under the rear hand guard) and as well as you stated, # 6 on the right bayonet lug. Now with the odd exception.....I most recently did find a 1950 LB, 6 groove with the above mentioned CMK 4....'58 dated barrel and #6 on the lug. It's serial starts with a 93L. I think this is a bit of a anomaly and not sure why that barrel was put on, unless it was changed later in it's lifetime. It does look however to have had the bolt serial # scrubbed and re-numbered to match the rifle serial #. Maybe this receiver was laying around at Longbranch for a number of years and then was finally assembled from various parts? Not sure scrubbing and renumbering bolts was ever done at the factory, I have my doubts......nevertheless a beauty and accurate and it does look time period correct.Two of the others start with a 93L serial as well while the last one starts with a 91L serial #.
Some things we will never know for sure!!!