A few notes:
+ The classic HW diopter as shown in my post above is no longer made. The last few years I think HW has sold a cheaper sight with a single clamp-type mounting "foot" which IMHO is not worth having.
+ If you find a used HW sight, be sure it has the correct rail spacing. HW rifles used 13mm rails up through late 700K serials (IIRC?...late 1970's), then switched to 11mm, but the sights otherwise look identical.
+ The Beeman sight IS a Williams, only the name stamped on it differs.
+ I've never had an issue with the FP-AG on any airgun. As you can see in my pics above, even on barrel-cockers with a bit of "droop" they end up on the lower part of the elevation adjustment scale.
+ The FP-GR can be problematic for HW airguns. The current model's mechanism hangs so low on the left side, that it strikes the stock wood, even on a smaller gun like the HW 30. It's really made for rimfires with 1" diameter receivers, where it will overhang the narrower stock.
+ The German auction site eGun is a fantastic source for sights, if you can get someone to ship outside of Germany! A search there for "diopter" is amazing. The infamous larger auction site can be a source too (especially a gent from Bulgaria with many interesting things). But, be patient and be able to recognize the sight you are looking for - many sellers call everything an "Anschutz!"