I would like to add that I have found this circle, or oval, containing one or two numbers plus a single letter, just about anywhere and everywhere on the frame, but only on the outside of the frame, never inside. I have also found the circle or oval with letters and numbers on the barrel clusters, on the shot barrel or on the barrel assembly lug block. While the letter within the circle always match on factory matching barrel and frame, the numbers never match, or at least that has always been my experience. These Date Codes are also present on the Model 94 frames and barrels, andI am sure on many others.
The marks on the inside of the frame, where the lower barrel seats when closed, are what I would call assembly stampings, and match frame to barrel, found on underside of shot barrel near forend lug, and also sometimes found on the butt stock under the butt plate. ~EE