In any event, you have a very tastefully done 12 gauge Model 21. You are in fact correct that the forearm checkering patter is in fact 1950's era. Engine tuning is not a difficult thing to do and was commonly done as an upgrade in the 1950's to everything including bolts on bolt acction rifles. So none of the items on your gun can point to Winchester and unless Winchester can up woth some record, that's all you will ever know.
Having a factory record on a field grade Model 21 is not important and one should judge the gun for what it is. There are many Model 21's for which no record exists. An in hands examination by someone with experience in Model 21's will render and opinion but that's what it will be, an opinion.
In the past I have had one 'Custom Built' that would not letter and various 'experts' had different opinions if Winchester or aftermarket. Tony Galazan had examined it at one time and said he believed it was factory and a few years later he purchased it from me.
Personally, I would not care if factory redo or a redo by someone else. There were some very good smiths of that era doing that type of work from Pachymar or down, its a nice Model 21.-Dick
