When I was stationed in Germany in the last 1990 and toured the Merkel factory in Suhl, I was told the 12, 16 and 20 were built on their own size frame. Simson's were built in that same factory.
Just measured my Merkel 16 (supposedly on its own frame) built in Jan of 1993.
Between the two firing pins (inside edge to inside edge) is exactly .917.
Across the side clips 2.14"
Across the action at the bolster 1.6
Depth of action .90
Compared to my Sidelock Hubertus 12 gauge also built in that same factory:
1.056 firing pin center
2.27 across the side clips
1.62 across the action at the bolster
.929 depth of action
Based on above I'd say it wasn't built on a 12 gauge frame, but I am comparing a boxlock to a sidelock. Hope this helps.
