The Stevens 425's and the Visible Loaders used a very narrow hole pattern for the peep sights, much narrower than the Stevens standard of 1.5", under 1". These were both designed by Edward Redfield, I do not know why his designs had the odd spacing. The 425's used a sloppy #8-40, the plug screws on one of mine measure a few thousands more than a standard #8. I tried a #8-36 but that is not correct, it starts but binds up. The Visible Loaders use a #5-44 with a large head the same diameter as #8 screws.
Lyman, Marble's and even Watson made tang sights for these. They must have made more than there are guns since they often are found 'new in the box'.
They are all drilled & tapped with two holes on the top of the receiver for a scope mount. Stevens had a special scope & mount for these that used a solid mount, the first listing describe it with internal vertical adjustment only, later as also having internal horizontal. The scope was only listed a couple years with some later catalogs having it over stampede as 'DISCONTINUED'. This is the only model scope Stevens made with internal adjustment. The scope has a long eye relief and mounts with the eye piece about even with the front of the hammer. The front mount uses the barrel sight dovetail.
Some tang sights -
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