The HW 55 with Williams sight was an "Air RIfle Headquarters" thing only. Robert Law at ARH had some special models made that were sized to fit the groove spacing of the air rifles he sold.
He loved creating exclusive models - and making up names for them! - by adding peep sights, scopes, factory upgrade stocks, etc. to standard guns. In their 1980 catalog you had this many choices for your HW 30:
+ HW 30 M/II, basic stock and sights, Perfekt trigger
+ HW 30 S, basic stock and sights, Rekord trigger
+ HW 30 S-X, basic stock, aperture sight, Rekord
+ HW 30 D, factory Monte Carlo stock, basic sights, Rekord
+ HW 30 D-X, Monte Carlo, aperture sight, Rekord
+ HW 30 C-X, Monte Carlo, Weaver scope and mounts, Rekord
In those days HW rifles had the old "13mm" rail spacing, and ARH had sport and target versions of the special Williams for them. My 55S has the "834T" sight. The "754 type foresight" mentioned in these catalog cuts for the HW 30 and 50 was a similarly customized version of the classic Lyman model 17A interchangeable-insert tunnel, with mounting dovetail and height tweaked for HW rifles (not required for the HW 55 which had its own tunnel sight of course).