AFAIK, (and I am often wrong!) a TRAP grade from that early should have 'Trap' stamped onto the bottom of the bbl extension regardless of the gauge.
But you just hate to say 'always' in this business,,410bore makes the point.
The 26" bbl, Full choke and raised matted solid rib (or plain matted bbl) would be correct for an early TRAP in 16ga.
Only the one bbl length offered in 16ga I believe,, w/Mod choke as an option.
Black Diamond stocks standard.
Stocks could be had in either straight or pistol grip. Fitted w/a rubber recoil pad. Most any stock dimension was done to the customers wishes.
Winchester brought out their own logo marked pad in the early to mid '20s.
I think the first pads used were Silvers Pads. Don't know if you could get them to put any of the others available at the time on the gun.
Without the 'TRAP' marking, I'd look at it very carefully.
Get a true M12 collector that knows the small details of these to look at it or the specs.
The TRAP grade was a semi custom Grade,,not necessarily a Trap Gun. Wood cut to customers specs, a couple of choke choices (even cyl avail in all 3 gauges), Different bbl lengths in 12ga, but no other choices other than 26 in 16ga and 25 in 20ga.
If you wanted more,,like engraving, you stepped up to the Pigeon Grade.
It could simply be a FIELD 16ga w/a solid rib that has had the TRAP wood added.
It wouldn't be the first time it's been done.
Solid rib M12's were available starting almost from the beginning of production,,1913 or 14.
Sounds like a very nice shotgun, The configuration & gauge with that condition would draw alot of attention from collectors if it left the factory that way.
,,,,but if it's not a real TRAP grade,,don't pay for one.
M12's are upgraded on a regular basis,,especially the smaller bores and rarer configurations. They just put a gun together to match what the factory would have built.
Orphaned high grade stocks don't sit for long at a gun show.
Alot of M42's are upgraded in that way to 'DeLuxe' and 'Trap' by upgrading the wood & stamping it w/individual hand stamps into the bbl extension thru the factory bluing,,,,just like the factory did.
Shockingly, there's no factory records available when the owner goes to check the validity of his new purchase.