Thanks, guys.

The mount is Mould's two-piece extended eye-relief unit for a Ruger 1S.

I got it from Brownell's back when they still had his 1S mounts. I don't know if they discontinued this one, because I can't find the 1-S mount listed there anymore. But, depending on the type of Ruger #1 you have, Brownell's can probably special order it.

The scope sits reasonably low and the eye-relief is perfect, with room to spare.

If I can find a 1-A in .303Brit, I may try and get a Moulds for it as well.