Not much I can add to this quest for information as I have not come across a similar device or patent relating to it before.
One possible avenue of research is the excellent series on airgun related patents that ran in Guns Review during the late 1970s, if anyone has the series to hand. That's assuming it was ever patented of course.
Kind regards,
John M
Currently looking for Baikal Makarov pistols with the following prefixes to the serial number: 98, T01, T09, T21, T22
Prefer boxed or cased but will consider loose examples too.