I think I might've found a likely lead on the identity of the library used in the late 60s shorts! Plugging in "Dehace" (the name of the library Conzelmann and Hanesch ran in the 50s and 60s) in the Internet Archive's "search text contents" function got me a 1959 Variety article announcing the formation of a library known as "International Music Library"/"New International Music Library", run by a director and producer named Marion Gering. The library offered music recorded in various European countries, but of particular note is the fact they had a branch in Germany, "tieing into the resources of Dehace, largest film music library in Germany". If this library was kicking around well into the 60s, I could easily see it being where Art Clokey found the Selected Sound tracks used on the show.