Markings on bottom of receiver
 

Can anyone tell me the significance of the numeral "1" on the bottom of my 12 gauge grade 2?
I don't even know what you call this milled out area- I know so little of hammer guns but I am anxious to learn.
It is a 1 frame gun - did they mark frame size there as well?

Could be frame size. I have never seen this before.Does the floorplate have a number one on it?

thats a new mark i have never seen before. we do have some early 1 frame guns with the #1 stamped in the wood under the trigger guard after the serial number.

George, I have thought about it and can't come up with any other reason significant enough to actually stamp the frame in an obscure location of something that other fitters and contractors would have need to know about the frame than the frame size.

It kind of makes sense to me. After all, what more appropriate place to mark frame size than the frame?
Obviously an experienced factory worker could spot frame size from 10 feet away, but....