I have this rifle (SMLE No 4 Mk 1) but have no idea what scope mount it is drilled and tapped for . . . anyone know what scope base this might have been drilled and tapped to accommodate? Thanks!

. . . maybe a Weaver Side Mount Base #1 (hole spacing is approximately .720/1.660/.720)?

Are all the holes drilled ? The one nearest the receiver ring looks odd , could be just the angle . I agree it looks to be a weaver .
The weaver is a 2 piece set up , the receiver mount ( base ) and either a long or short side mount ring set is available . Tasco also made a side mount similar to the weaver but not interchangable.

Yes, all four holes are drilled and tapped. The size is 8-40. I sure think it may be a Weaver set up. Tried to Google what size screws Weaver uses on it side mount bases but could not find anything.

The mount was in fact a Weaver side mount. I have never used a Weaver side mount before, it is actually a pretty nice set up. It allows you to use the iron sights but is not as high as most see-through rings I have seen. Now time for a range trip.