egafd.com

The (step?) father in Hans Billian's 'Josefine Mutzenbacher, Teil 1' is credited to Frithjof Klausen. This actor was one of the most prolific of that era, yet there is little information about him. Did he usually use a different name?

I am still doubtful that this is his name - it is such a North-German name, for someone playing South-German parts. It's a bit as if someone named Huw Llewellyn played kilted Scotsmen all the time.

I think the EGAFD identification is based on a Teresa Orlowski tape.

One possible other name might be "Walter Schwarzmayer" - the name appears in the credits of "Gaudi in der Lederhose", and he appears in that film.

I have to agree with you. Notwithstanding the fact that he appeared in many porn films of the late 70's in leading roles, no records of him seem to exist. It is far from clear who he was. Given the fact that details of other male stars of the time, e.g. Sepp Gneissl & Siggi Buchner are readily accessible, is there a plausible explaination?

Details about Gneissl and Siggi Buchner are readily accessible, but that is NOW, with Internet and all. 25 years ago was a completely different story.

People were cautious about being identified as a porn actor, both male and female. There was even a legal side to it, because porn had only been legalised in Germany in 1975, and people were not quite trusting this new freedom. After all, Christine Schuberth had been briefly jailed in the early 1970s for speaking (!) the Josefine Mutzenbacher part for an LP.

Back in the seventies the only way to watch a porn flick was to go to a specialist sex cinema, so the risk of being identified by acting peers or family members seemed relatively low.
Still, in some of these early porn flicks you can see an abundance of wigs.
For example, in "Mit Gurke und Banane", Karin Lorson not only wears a wig for her penetration scene, her head is underlit so that it's difficult to recognise her.