I added some close ups of my mounts Gene.
Joe
Joe, I finally got some more pictures, starting with the 3/4" with manually adjusted center mounts that attach to the barrel, the front is in a screw on dovetail piece but I think it could be put right in a dovetail in the barrel. The rear mount could be attached to a bracket for an 1899, they're all just held on with a single large screw and can pivot.
Manual adjusted mounts -
3/4" center mount, micrometer adjust -
Joe, the front mount is later than yours but should be similar, just that it has slotted adjusting screws instead of thumb screws.
5/8" side mount, micrometer adjusted -
The tapered hole in the top of the rear mount is for a lathe center, for tuning the elevation shaft, that shows how far the ring is off center for an 1899. If you see a side mount with that tapered hole near the center of the ring it will not work on an 1899 with a front mount with the usual offset, I think all front mounts might be the same offset.