Interesting - Baker's 4 bore was 21 pounds. A gun this heavy would never be carried by the hunter at that time.

This gun above predates Forsyth's time as the lock on the piece dates it to a mid to late 1700's lock - so the 'timing' noted is quite accurate.
Quite possibly though, it is a fowling gun, not meant for game.

By the early to mid 1800's, 16 bore through 12's were the favourites for hunting elephant, buffalo, bear and tiger, however some used heavier bores to 8 for specific use, as in exploasive shells or for 'stopping'.
The natives of that period were using the larger bored guns but as Forsyth says, they were matchlocks, not English guns.

It would be an expensive venture to entice me to fire a 4 bore, let alone a 2.

Nice but plain (as it should be) chunk of English used for the re-stocking job, but should have been stained, I believe.