Much then would depend on the quality and fitting of the forend and the ambient humidity, as well as the fitting of the backsight guard and the single woodscrew which attached it to the forend!
It would be interesting to know why something like Parker's proposed design was not used, since it was both simpler and cheaper to fit and easier to use. A sprung steel band around the rifle barrel would have made it even more serviceable, without drilling and tapping. Perhaps they didn't have the connections to get their design considered?
Looking at the spacing of the bases in the illustration, they could have made a short base to go in the rear sight mount and then screwed the rear base to the Nock's Form or even on the "receiver ring" as we call it here in N.A. This is what the Canadianarmourers did for the Ross: screw on commercial bases with spacers made to suit. They could even have used the rail from a telescopic layer and added an extension to locate the rear base for fitting, thereby transferring the zero of the iron sights to the telescope, within reasonable parameters.
One advantage of the sliding Winchester scopes was that mounted over-bore they could probably be slid forward far enough to use a charger to load the rifle, whether a Ross or a SMLE. The sniper who objected to pulling the scope back could probably have rigged something up with those big rubber bands that were sometimes used to hold the W&S scopes in approximately the same position on their ill-designed mountings.