Back in the day, Parker Hale made a side mount system - the A29 for the No. 4's and the A27 (I think) for the No. 1's. A29 has a flat side, A27 has a curved side. Essentially an 1/8" piece of angle aluminum that had a one piece scope ring base that screwed onto the top of the A29, then the Parker Hale rings attached to that. I have heard of people using the A29 base and existing holes, but then putting a weaver or picatinny base on top and using weaver style rings. These were made by Parker Hale for their 303 conversions that had the rear sights and cross bridges removed. The side has three holes for mounting screws and a "cut out" to fit around the ejector screw. I just sold an A29 setup - the side mount holes are spaced at about .850" centre to centre, and according tot he buyer, his side holes were 6x40 threads - this would be the screw size used on Ruger Scope rings, not the usual 6x48 used with weaver and leupold mounts. The lower RALS.3 rings work well with the A29 because there is no cross bridge or rear receiver sight ears to get in the way - PM me if you looking to go this route.
As well, I have a Pachmayer side mount "swing away" mount for a No. 4 - this was also mounted to the side of the receiver - is essentially a big hinge that allows the scope to be swing into position over the bore, or swung outboard to allow for charger loading or using the barrel mounted iron sights
