The Marbles sights for these are marked V7, these have the same base as the V6 for the Visible Loaders but the V6's are about 3/8" taller. Measuring (2) V6'S, (2) V7's and (1) Lyman HP I come up with a hole spacing of 13/16", but the actual measurements varied from slightly under at 0.808 to right on at 0.812. I do not think this spacing is used on anything else. The clearance holes are the same on both the Marble's and are oversized even for the #8 screws at 0.193", so they position off the head of the screw making the correct size important. Standard screw diameters are; #5's - 0.125" and #8's - 0.164" with the oddball for the 425's about 0.168" measuring two original plug screws over the threads which usually runs slightly under the specified diameter.
The Lyman HP is the best one to get, it has the post centered between the mounting screw so it sits farther forward allowing a comfortable thumb position and allows the post to be much shorter. The designs of the Marble's and Watson do not allow for that narrow of hole spacing putting the sight in back of the holes.
The #5 screws for the Visible Loaders are the same size as most other Steven's except the 425's and there are several places that sell them, I think Apertures 'N' More is the cheapest -
aperturesnmore The ones I bought have rounded heads that measure 0.251" and have very thin slots, 0.250" seems to be the standard on most tang sights and most tang sights also have oversized clearance holes so they position off the head diameter. For #8's I've found the lower price screws you get today, like in the Brownell's kits or the ones offered or ACE Hardware, have flat heads that run about 0.240" with very wide slots that do not look 'vintage', the heads have a slight bevel so they still center in the holes. The screws in the Pachmayr kits are better with thin slots and rounded heads, but still they run a little under at about 0.245" and they do not seem to sell them other than in complete 277 piece kits which contain more #6-48's than #8-40's.
