Does the revolver look like it may have been refinished at one time?
Perhaps if so, part of the mark, that outside circle was polished away/lost in refinishing. Other proof marks may have been polished away as well.
Just a wild guess. Tough to tell from the one pic.
It's the London Proof Hs 'View Mark' for a civilian firearm being proofed
It must have been an extremely busy time at the Brit Govt proof facilitys then.
Likely some short cuts were taken at times like any other factory during high volume repetitive assembly line type work.