The frame size of a Parker is marked on the bottom of the barrel lug.  It represents the relative size and weight of a Parker's frame and controls the size of both the barrels and stock that was fitted to it.  The fame size was, and still is, determined by measuring the distance between the firing pin centers; the frame size designation is the number of 1/16th of an inch from 1 inch.  For example a zero frame would measure exactly one inch between the firing pin centers; a number two frame would measure 1 2/16 or 1 1/8 between centers.   There is a correlation between the frame size and the gauge of gun it was used on as seen in the chart below.


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