I have two Webleys, a Mark 1 and a Mark 4. Mark 4's were only produced from 1913 to 1915, so the Mark 5 could be pretty old. I haven't read the research in as while, so I can't remember when the Mark 6 changes came down, but I believe it was before WWII, rather than during the war. The Mark 6's only changes were a square butt, and a six-inch barrel, IIRC. For the life of me, I couldn't tell you the difference between a 4 and a 5, as I didn't pay that much attention while doing my research, because I don't have one.
I have no clue about websites. Try a Google search with Webley, Webley-Pryse, and Webley-Greene, and maybe Royal Irish Constabulary, i.e. RIC revolvers.
The W-P and the W-G were the guns that directly preceeded the Webley as we know it in the 1870's. The Webley could be regarded as the English Peacemaker.
Most of the marks are military acceptance stamps, and the NP stands for nitro proof, indicating your gun was proofed for smokeless powder.
If it has big holes, .455 it is. Good luck finding ammo, though. Fiochi still loads it, if you can find it. Last time I saw some for sale, it was $30/box mail order. Both of my guns were converted to run on .45 ACP in moon clips, or Auto-rim. (Actually, the Mark 1 won't take Auto-rims. Only the outer half of the cylinder was faced off, so it only takes half-moon clips.) I think you can make cases out of Auto-rims, but that requires thinning the rims down. All the English cartidges have very thin rims compared to American rounds. If you wind up reloading for it, use lead, rather than jacketed bullets, as the metallurgy in pre-war guns is a little primitive, and jacketed bullets are supposed to prematurely wear the barrel out.
If you did a search, I trust you read about My Curious Webley Revolver? That thread is chock full of neat stuff.
Springfield Sporters has some Webley parts, if you need 'em. You can get a Mark 6 barrel for $30 that'll drop right in, should you desire a six inch barrel. If you can find a spare cylinder, you could have it converted to .45 ACP. Switching cylinders is pretty easy on the Webley.
I hope you can get it running. My Webleys are among my favorite guns, and they're reliable, accurate shooters, with quite mild recoil, and smooth triggers.
Good luck!