I'm not sure.
I bought it from George Caswell a few years ago. It was set up for long range BPCR with a long 45-90 Metford rifled barrel which I think was made by Snover.
I would guess that by the size of this action that it was originally a 500, 470 or maybe even a 577.

I will for sure shoot reduced loads in this gun although it does weigh 10lb 7oz which is right about what a 500/450 or 500/465 Holland Royal double weighs. I don't know if you can get much more weight in a falling block without adding a quarter rib or really heavy barrel.

Here are some weights of 1902 Webleys from the net.

400/350 8lb 4oz
375 Flanged Mag. 8lb 9oz
450NE 9lb 10oz.
450 NE 10lb 1oz
577 (100/750) 14lb 10oz