More gadgeteering from the dark recesses of accumulated stuff...and my sketchy old memory.
Back in the day, Robert Law at Air Rifle Headquarters in WV commissioned Williams to make airgun-specific peep sights. These were similar to today's FP and GR series, but with fixed mounting rails mating with each brand's dovetail spacing. They are secured by setscrews on top, to snug the sight up tight against the grooves.
This is the "ARH 624," a sporting peep sight (Weihrauchs had wider 13mm grooves back then, so the ARH peeps are a rare alternative to HW's own match diopter of the day). Seen here on an old HW 30:
And here on a vintage HW 50 is the "ARH 834T," the target version of the same sight, adding finger-knob adjusters and a taller elevation arm.
Next, a challenge - a BSF S54 Match underlever. This eccentric design has: A) relatively low front sight; B) barrel set below the top of the receiver; and C) high rail for the rear sight...so any rear-mounted sight must adjust VERY low. BSF's match diopter works of course, but is fragile, adjusts coarsely, and doesn't take German eyepieces! ARH also had custom Williams sights for BSF barrel-cockers, but they won't go low enough for the S54. The one sight that WILL go low enough is the current FP-GR, but its tall elevation arm strikes the gun's wood (note: the FP-GR may work as-is for those versions of the S54 with narrower stocks).
The solution was a mix'n'match. The base is from a brand new FP-GR, with the shorter top from an "ARH 833T." A tidy setup, with the bonus of better precision than the OEM sight.
And a surprise bonus...the other half of each sight was not wasted. The 833T's rails are about 11mm, so its base plus the FP-GR's top gives a 1980-vintage HW 35E (with 22-inch barrel) a sight radius that's almost silly.
Here is the whole motley old crew with their new "eyes," ha.
Here