This appears at first glance to be a BSF S-20, but further examination reveals that it is not.
There is no "S-20" markings on it anywhere, only "Wischo KG", along with the "Wilsker & Co." importer marking.
It clearly does not have the characteristic double impressed rings at the breech, and the trigger mechanism does not protrude from the rear of the wood stock. There is only one retaining pin for the rear plug.
Overall length is ~ 12.625", and the barrel is ~ 6.0" long.
I have not been able to find any pics of a similar pistol online, (perhaps I am using the wrong search criteria?), but it does not resemble any S-20 pics that I have seen.
I found the reference to the Wischo L50 in The Encyclopedia of Spring Air Pistols, but unfortunately, there was no pic of the gun, but based on the written description, I am wondering if the "KG" was a rebadged version of the L50.

https://vintageairgunsgallery.com/image/4j8Fh