I think I can explain your .935-.940 measurement. I seemed to remember something I had run across years ago when I was collecting chamberings.
The Brits had a total of 9 different “military” .45 handgun cartridges starting in 1868 with the .450 Adams and ending in 1939 with the .455 Webley Cordite Mark VI. I can’t find the document right now, but I remember reading that all of these cartridges could be fired in any of the Webleys (with backward compatibility.) In other words, any given Mark of Webley could fire any cartridge that preceded its introduction. The only caution was that, even though it would fit, it wouldn’t be prudent to fire a 1939 Cordite MkVI in an old black powder MkI.
Now several of the older BP cartridges had a .86 - .87 “ case length. In addition at least one appears to have a heel seated bullet so the chamber would need even a little more length to accommodate this.
If I can find my chamber cast metal I will try to take a cast of my unaltered .455 1st and get some accurate measurements.
I agree that it sounds that he may have a standard .455 chamber that hasn’t been altered.

__________________
Dean
SWCA #680 SWHF #446