Very seldom, if ever, was frame size recorded in the Parker Bros. production records. One can't even make a good guess by looking at the weights of the guns. As a case in point, my Grandfathers 1890 vintage 12-gauge PH-Grade was built on a 3-frame, but has barrels that swamp very quickly and weighs 8 pounds 8 ounces. I have an 1889 vintage GH-Grade on a 2-frame with very straight heavy barrels, that weighs a bit over 9 pounds. The Brothers P did more weird and wonderful things with frame size and weight and balance, to the point that even a 6-frame 12-gauge is known to exist --
In the later years, most of the Parker guns made for the 12-gauge 3-inch high velocity loads (Western Super-X, Winchester Super-Speed, Remington Arrow Express and Peters High-Velocity) were built on the 1 1/2 frame.