Hoot,
These devices are not used as the rear sight; they are used with the normal rear sight to "sharpen" the sight picture. They are sometimes confused with an "ocular" which does have glass but does not qualify as a telescope. Schuetzen rifles are not allowed to have telescopes mounted but older shooters sometimes use an "ocular" to allow them to focus on the front sight, without magnification. An "ocular" is rigidly mounted on the Schuetzen rifle and does not fold back into the stock. The devices in question would be used on hunting rifles. As always, I might be mistaken.
Mike
