Jason,
I don't think mfg date had much to do with the frame/barrel dimensions. The frames were still being shaped by filing and the end-use (quail, pheasant, duck) for the gun dictated the barrel weight and thus dimensions. Some of the 20ga guns were lightened to the point (5-1/2lbs) that frames were cracking. The breech balls could be spherical to almost flat blades. Besides the firing pin spacing you need to consider the distance from the barrel flats to the pin centers also. A LOT to match up.
Wish I had a Flues 16ga (working on that). I do have a Minier that I could check.
John
