I too have never heard of a Military association for the Mk.111 Supertarget, or one with a scope rail, which means it hasn't got the name 'Supertarget' on the cylinder.
I've had a few through my hands including an early prototype model, but none had a scope rail.
If it's not in Chris's book, I would be very suspicious about it's claimed association.
Anything is possible with Webley, as, at the time they would configure a rifle to the customer's specification, for a price.
Are there any pictures available ?