When I first read your post my guess was going to be an early gun. I have some early 10s and 12s that do not have frame size marked on the barrel lug. So yes that was the case with early Lifters.
I looked at the gun, which is a nice early Grade 2 equivalent Parker. I would "guess" a production date 1873 or 1874. The chambers usually measure 2 5/8" on an early 10 gauge lifter like that though they may have be modified at some point.
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