You might try looking for a Bomar rib I think they used to make one but you will have to drill and tap if you can find one, I know they made 6" ones, john traveler is right, the key way cutter for the elevatation screw will be quite a bit as cutters are high priced. I just checked a 1996 Gil Hebard catalog and Bomar made a 4" rib called a winged cover up rib for a heavy barreled 4" gun for a 10, 13, 64 H.B. 65, 547, 581, and 681.