There's quite a few photos of A5s on Ross MkIIIs around. Most well known being the rifle from the Winnipeg Rifles Museum being held by an NCO in 1940 or so. That photo is in Without Warning No photos I have ever seen of the bases alone, but they probably used commercial bases fitted directly to the receiver ring at the rear and a spacer block between the base and the barrel at the front. You can buy a brochure on the A5 scope off ePay that gives you the directions for spacing etc. According to one or two accounts this was was the most common scope & rifle combination used in the CEF right to the end of the war. The scopes were fitted by unit armourers in some cases, perhaps most cases. The MkIII is usually a very accurate rifle so no selection by test was probably necessary.
The barrel was usually cut off behind the front sight base as it must have obstructed the FoV through the scope. In some cases the front sight base was probably just driven off, but it's surprising how many of them seem to have been cut. Perhaps the 30.5" barrel also obstructed the FoV a bit, making the cut necessary.
I've heard there is a gent in the USA who is cleaning up at service rifle matches with a MkIII and A5 scope setup.
