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It's almost common knowledge that Karin Schubert appeared in her first hardcore film in 1985. However, Private magazine that featured her in a hardcore lesbian set in issue 13 (circa 1968, with her real identity and exact biographical info) presents a short report on her cinema career in issue 29 (1973) accompanied by screenshots from a film where Karin and another blonde give head to a guy. Any idea where this may had come from?

Funny and curious you should mention that.
I recall reading French magazines in the mid 1970's wherein one of the writers who was commenting on the mainstream movie career of Karin Schubert inferred that there existed evidence that she did hardcore (I don't recall whether he meant movie loops or magazines), and that he would provide more information and evidence. The magazine in question folded not too long after that and the matter was left in suspension.
I have heard similar rumours after that but I never saw any evidence. However I believe Karin did pose for hc photo layouts in the 1970-1980's in several Italian men's magazines, but I do not know what the timeline or publication date of those were,and whether they were done before she actually took the hc movie plunge.
Is there anyway to see those pics/photos you speak of?

It's in the issue 29 of Private. I obtained it through so-called "illegal filesharing" as a PDF file.

The inside cover of Private Magazine #29 carries a copyright notice of 1974 (I suppose it is credible enough). On page 24 of that magazine are reproduced about 5 small pics of hardcore action between two women (one of which is clearly Schubert) with a male. The written article refers that these pics were taken from a sex film, most likely a film loop which were prevelent in those days (however no date in indicated when that sex film was shot, but I would suspect somewhere in the late 1960'or or quite likely very early 1970's before her international mainstream career took off).
I have a feeling that the writer who wrote in that French magazine in the mid 1970's may have just seen that Private Magazine 29 issue and may have been making (indirect) reference to it.
However an additional trail must be explored/pursued as Private Magazine 29 refers to an earlier photo-lay out (a lesbian one) in Private #13!

OK. Private Magazine #13 does not carry any copyright notice, and there is a long and large photo-layout between Karin and another blonde.
In the written article there is mention that Karin had recenbtly made a couple of movies in Germany and had completly a couple of movies in Rome. Nothing specific is mentioned but I am supposing the photo shoot was most likely made around 1969-1971.
With regards to the sex film mentioned in Private #29, don't know what that is, but only oral sex is shown in those 5 small pics reproduced in page 24 of that magazine. Don't know whether the sex action involves other things other than oral.

This gives her one of the longest careers in the biz [admittedly non-continuous],
at over 25 years.

And that raises the question as to whether there were other "compelling" reasons why she began doing hc movies in the mid 1980's, other than the ones we heard about money and family problems (with her son)....
She had been doing mainstream movies since around 1968. Nothing really major "A" class stuff mind you, but must have paid better than flipping hamburgers at the local diner.
So by the mid-1980's (after approx 17 yrs in the movie business) the money well was so completely dry that she had to "resort" (kicking and screaming) to hc movies? Or is it something that she was attracted to and she had no aversion to, based on her prior soft core and hc core modeling work?
And if we are speaking of records, I think perhaps the longest and most continuous hc career might belong to Sharon Kane.

Eventually, the outcome is no one saw or has that film.

It is most likely an 8mm loop lasting only a few minutes. Maybe some collector may have it or may not know/realize that Karin is in it. (And who knows maybe she did more than one loop or photo lay out and nobody realized it at the time). I wonder how Private was able in 1974 to come up with that evidence, or even remember Karin appeared in an earlier issue (#13) of their magazine. Somebody there had a sharp mind and memory.
The fact remains she did hc work more than 10 years before her "formal" debut. She was no stranger no porn.