Welcome! According to the Parker serialization book, your gun is indeed a GH grade (which is the same as Grade 2 -- Parker used a parallel letter/number system.) that left the factory with 28" damascus barrels and a pistol grip stock. So the G and 2 are really redundant.
The D on the barrel flats is for Damascus barrels. The C is not known. As you already correctly identified, the 3 with the superscript 10 tells you the barrels had an unstruck (not completely finished) weight of 3 pounds, 10 ounces. The 1 1/2 on the bottom of the lug does represent the frame size. The 1 1/2 was a common size for 12 ga guns.
Have fun with that family heirloom! ---Matt
|