I believe John Andrewski included those mounting bosses/nubs on a number of builds that he did, together with a matching section of picatinny rail -- but they are not a standard in any other sense. I've seen some other threads where folks have indicated that Andrewski doesn't have any of the mounting rails available for sale (although I suppose it never hurts to ask).
I'd say your best bet may be to find a local gunsmith/machinist who's willing to take on a small project and make you a rail from a picatinny blank. The Andrewski rails are radiused to fit closely to the tube. If you didn't mind a bit more height over bore, you could forgo the radius and it'd be a really simple project. Perhaps even have a block of steel machined to fit the mounts w/o a radius, then send it off to one of the people who machine pistols slides for direct mount optics. With a direct mount, you'd likely end up with the same height over bore (if not better) as the "correct" rail plus a pictatinny adapter.
In any case, a red dot is a very worthwhile addition to the Sterling; I was lucky enough to get my Andrewski/DLO Mk4 build "new in box" with the picatinny rail section still in plastic. Good luck!