Jack,
Bore dimensions of 0.730" to 0.732" are nearly spot on for the precise bore size (0.729") of the "perfect" 12 gauge. This is because 12 lead balls of precisely 0.729" diameter weighs exactly 1 pound.
The chamber dimensions of your gun are correct for the Parker 12B brass shot shell (0.765-0.770") but too small for the Parker 11B brass shell (0.790"-0 .800") or the Parker paper 12 or the Parker or UMC 12A brass shell (0.805"-0.810") which is the same as a modern 12 gauge shell. (Modern 12 gauge shells are too big to fit in your chambers. If you want to shoot your gun you will need to get some Parker or UMC 12B shells and hand load them.)
Hence your gun is a (very slightly over bored or very lightly pitted) 12 gauge Parker that was chambered for Parker 12B brass shells.
I am very much interested in hearing more about it. Could you please let me know the serial number, grade, what it says on the barrel rib? If you send me your e-mail address I will be happy to send you a copy of my Double Gun Journal article from the Winter 2008 Volume that contains all of these dimensions for the Parker chambers, bores, and shot shells.
Yours,
Richard B. Hoover
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