1 1/4 Oz was the normal proof for 2 3/4" chambers.

2 1/2" chambers were usually proofed at 1 1/8 Oz.

As an aside, it looks like your gun was originally a 13/1, so would have originally accepted a 0.719 plug gauge, but not a 0.729 plug gauge.
This was quite usual at that time, but can have a nasty 'knock on' effect - in that if it now measures greater than 0.729, it would (in the UK) require reproof before sale. It is quite common to find guns originally proofed at 0.719 re-proofed at 0.729. I have one that has had that done.