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The digit to the left clearly has a tail on it like a lower case "t", which the "1" on the right does not have. I don't think the left digit could be a "1", so I don't think it's an 11 bore stamp.
That's your prerogative but get a set of tubes from the period and look at the stamp. Usually it's two "1"s with one inverted with respect to the other and facing. Pull the image into some image viewer, zoom in and rotate it to see all perspectives. The left digit is more of a "j" in this case due to wear or pitting. The location and orientation is typical for the period. Look at the "1" in the "1 1/4" above it and consider that there is wear or pitting at the tip of the left, inverted "1" possibly by a worn die or ill struck "1". It's the stamp for a 11 bore.
Kind Regards,
Raimey
rse
