David is correct Milt- the Trojan was only offered in 12-16-20 gauges, never with ejectors, most often with double triggers, and without the 1878 patent date forearm latch mechanism found on the Graded Parker hammerless guns.
But as they say, never say never too often- Peter Johnson in his 1960 book on Parkers mentions a very few Trojans made in 24 gauge- on the PGCA site I also read about a few Trojans offered with Twist barrels instead of the Trojan Steel-
I agree, grade numbers on the watertable, but the frame size for your Parker is found on the bottom rear part of the lug-if a later series gun, right behind the pinned in place tool steel wedge shaped wear bar-
Almost always in the Trojan- 12 gauges were No. 2 frame, 16 gauges were No. 1 frame, 20 gauges No. O frame size. The difference in the size of the frame in the same gauge Parker is mainly the dims. on center between the firing pin holes in the standing breech face- If you had two 12 gauge VH(E) Parkers, one on a size 2 frame, the other on a size 1 frame, you would see the difference in that set of dimensions right away- also the wall thickness at breech of the size 2 framed gun would be somewhat greater than that found on the size 1 framed gun--
I have a shooting companion that inherited a "matched pair" (sort of);

two 12 VHE Parkers- both on the No. 1 frame-one made in 1913 with 28" Vulcan steel barrels and straight hand stock and DHBP, one made in 1924 with 30" Vulcan barrels, pistol grip and a red recoil pad- both balance perfectly at the hinge point and weigh aprox 7 lbs. He is wise enough to use RST 1 oz. light loads in either one-