I do not see a separate number on the one I have. It looks to be identical to the same part on the A27 mount for the No. 1. If you are not worried about authenticity, and just trying to get the thing to work, you can buy a flat bottom Weaver #50 base, or similar, and drill it for the holes for the base screws, then use weaver style rings. Just a note of caution - the base is aluminum, and most of the used one that I have received had one or both threaded holes stripped. Use good sense when tightening those two screws. In those cases, I re-drilled and tapped to 8-40,and then opened up the holes and recesses on the top bar, accordingly.