.360 BP Express.....NOT ".360 Nitro Express"......NOT 50 Cordite.

Ah, so there, thanks Ben, I didnt give a close look to the picture of the chamber cast.
That explains a lot because the original .360 Express (.360 2 1/4") was a BP round originally 50 grains black powder and 150-195-215 grain bullet. Therefore the 50 - 200 marking.
The old catalogs show the Nitro for Black as 22 grains Cordite and 190 grain metal base (like a taller gas check) bullet.

(The specific ".360 Nitro Express" nitro proof would have been marked 30 Cordite and 300 grain metal jacket bullet)

Agreed, the round "bolt hole" dropping block small action 1902 Webleys are not a Nitro Express deal. It likely would contain the pressure but would get battered to death in short order.

Decent looking job of a refinish on the action; the originals action, breech block, and lever would have been case color.
Maybe underwent reproof and so marked when it was restored?
- Mike