Hi Mike, the one I had was stamped underneath, but not 10 A.S.!.....It was a very different shallow profile, but the photo of it I saw did not show this well. I think it was for a Marlin or Winchester from memory, so had a channel underneath of the same width all along. It would have been possible I suppose to weld and re-shape it, but the angle of the sight arm would never have been right. I think they made different castings for the bases, and while it may have been the case that some could be common to different guns and could be modified, it was just my luck that I bought one that was totally as wrong as possible!
I wish I had kept it now, as It would have been possible, but a lot of work, to make a complete bas from scratch.....lots of filing and shaping, but possible.