Overload proof marks usually mean a post WW1 mfg. gun. JG is the stamp of Jim Geary- head barrel man- sometimes you'll see a K or WK- for Walter King- plant Supt. and son of C.A. King- The barrel markings on the rib should match (in many cases) the grade number stamping on the receiver flat- a 1 is a PH(E) with Parker Steel, a 2 is a GH(E) with parker Special Steel, a 3 is a DH(E) with Titanic Steel- if you have a VH(E) or Vulcan grade (introduced about 1905) there will be No. grade number stamp, as 1 was already used by the PH(E), next higher up grade- and discontinued in about 1927-- If the barrel rib roll stamp is Trojan Steel, you have a Trojan- the economy grade introduced in about 1915- and the stamping on the water table will usually be a O inside a Diamond shape-
Trojans were the basic entry level Parkers- no ejectors, forearm latch, different frame sculpting, some later Trojans may have a single trigger (usually Miller)--But know this- all the internals for the same gauge and grade Parkers had the same "guts" or internal mechanisms- quality workmanship-- All the fancy barrel steel names (except for Sir Joseph Whitworth barrels from England) are the same metallurgically speaking- a high nickel ordnance grade steel--
You will also find a number stamped on the bottom of the barrel lug- gun has to be dis-assembled- All 12 Trojans were made on the No. 2 sized frame, so that number will be stamped on the bottom of the lug-- some VH(E) were on that frame size, some were on the 1 & 1/2 size, a few on the 1 frame--
All this pertains to strictly hammerless Parkers with Nitro proofed steel barrels- the Damascus and hammer guns- whole 'nother ballgame- My guess- you have a nice old 12 Trojan- I have owned two of those over the years, and they performed flawlessly- and one other thing about the Trojan- it can't be 'counterfeited" or falsely "upgraded" into a higher grade Parker to fleece the unwary woul;d be buyer- the frame shape is different- Enjoy!!! RWTF