Folks, thank you the superb info.

As you probably guess, I'm planning on building up a recreation of a CEF sniping rifle. I will probably modify a set of blocks left over from a batch that I commissioned a good few years ago that reproduced what Winchester supplied to the USMC for M1903 before the Great War. The spacing on the M1903 was 7.2" which gives half minute graduations on the A5 rear mount, but with the rear block on the Ross being mounted relatively further forward, I think I may have to use a 6" spacing to keep the correct eye-relief.

I'm self-isolating at the moment so spending some time trying to finalise plans on a host of projects (classic rifles and vintage cars) that have been hanging around for too long.