All very interesting, but if the focus of attention for the gun in question is a manufacture date of 1872, it should be realised that choke boring was only just being introduced. Proof marks at that date IIRC were principally stamped as bore gauge - and also muzzle gauge if barrels were choke bored. The word "choke" was stamped at a later date, choke designation having earlier been accorded the words "not for ball".
There are many references in the early 1880's to heavy loads used in trap shooting, in both 12 and 10 bore guns ( eg Capt. B and Doc C) - frequently heavy powder charges were matched with light shot loads in both 12 and 10 Gs, as also were moderate powder charges and heavy shot loads ( although 1 1/4 oz was max load under some rules). However these notes do not clarify matters on the question in hand.
So with the only information provided, and lacking a note of the actual proof stamps on this gun, the bore and muzzle measurements should give some clue to date of manufacture and intended use and by inference the original chamber length. Bear in mind that Greener, Scott and possibly Westley Richards were the main suppliers of trap and sporting guns to Nth America at this date, (and a few guns might ? have had specific chamber/bore/load stamps) but the great majority would carry the bare minimum of proof marks on the standard model exported to The Colonies.
Greener 1st Edn does not seem to specify standard - or even non-standard chamber length, however he does make reference to the powder available overseas being inferior in strength and by inference that larger charges may be necessary to achieve the desired performance ! - so presumably chamber length may be increased on specific instruction by the purchaser.
Perhaps also information on the lock design would help the guesswork here ie non-rebounding or rebounding ?
