I have observed markings under the butt on various guns as well. Usually one will find the last 3 of the serial number on both the wood and the plate. But I have seen sinilar markings to yours as well. Both that I recently saw were on hammer guns. The one mark is certainly the drop at heel.
Yours look to read 1, 2-1/2 and 1/4. I wonder of the 1 is the stock size (which your butt size is a 1. And 2-1/2 could be the drop at heel and I am not sure what the 1/4” may be. Does it have any cast?
See here photos of the ones that I mention above. These guns were completely original.
The one is clearly 3-1/4. The large white is the serial. I wonder what the RB is on the one. ?
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