Jack,
You need to get measurements of the internal diameter of bores of the barrels away from the muzzle and the chambers. If the gun is a 13 gauge, then the bore diameter will be between 0.702" and 0.719". The muzzle diameter is not suitable because even very early Parker's had constricted choke bores. The chambers are not suitable because the gauge size of a shotgun is defined based on bore diameter. As T. D. S. and Capt. J. A. Purdey in their wonderful 1936 book "The Shotgun" explain on pg. 141---"The gauge division 12/1 includes any bore of diameter .730 to .740 inclusive; and the gauge division 12 any bore of diameter from .729 to .720. If the diameter is .740 the gun is not a 12-bore but it's gauge size is 11, and the gun is proved as an 11 bore."
They go on to say: ,"The gauge size of barrel is always taken at a point 9" from the breech end."
If I can be of any help, please let me know. If you have captured a 13 gauge, it would be a great find indeed.
Richard B. Hoover
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