Just my opinion, but if its a difficult to find caliber, the value goes down. Many other potential buyers feel the same way. I like New Services, and there are idiosyncrasies with those British calibers, like some are converted to .45 ACP and such. I just don't try to keep up, so pass if they're not .45 Colt of .44 WCF or Special.
The cylinder looks like it's better finished than the frame, but it should be the opposite. So probably the above mentioned conversion has occurred. Finish level looks about 75%. But it also kind of looks like the frame was reblued, but hard to tell with the pics. I see a British stamp so it's a WWI contract gun, likely.
50 years ago these were very cheap, and people tried to get it where you could shoot them. Value goes down if it's not original. A set of grips would cost at least $100, so subtract that. . If it were for sale locally for $300 I'd buy, $450 I'd maybe buy it. If $900 I'd walk by. I say that because for every collector with deep pockets that will say "I'd buy every one for $500!" there are 25 collectors that won't. And if you are buying as an investment, that means it will be slow to sell, the higher it cost you.