This is "Provide Some Answers," which is a feature where long unresolved plot points are eventually resolved.
Our story begins at a strange period in the life of any comic book series, the last few issues of a creator's run when they are not intending on the issues being the last few issues of their run (the most confusing time in any comic book is when a writer is abruptly taken off of a series, as the writer has not had time to draw his/her own plots to a natural conclusion, so the new writer has to do that and very often the new writer will take things in a COMPLETELY different direction than what the original writer had in mind).
Anyhow, during his brief run on X-Men, the great Arnold Drake introduced a new mutant named Lorna Dane, who had magnetic powers. She was captured and taken under the control of a new mutant villain known as Mesmero (who had the ability to, well, you know, mesmerize people. It's right there in the name). Mesmero was working for a mysterious boss.
At the end of X-Men #50 (drawn by the legendary Jim Steranko, with inks by veteran comic book artist John Tartaglione, perhaps best remembered for his romance comic artwork at a few different companies before he became an inker at Marvel in the late 1960s/early 1970s), we see that Mesmero's mysterious boss also claims to be the father of Lorna (now known as Polaris)!!!
Okay, suffice it to say that the whole Magneto/Polaris dad thing has been all over the place in the years since. Even in that original story, before we knew anything more about this Magneto, Iceman supposedly proved to Polaris that she wasn't Magneto's kid, but it's funny, since his proof wasn't really much proof at all. But anyways, maybe I'll address this in the future. Probably not (since I've already done an Abandoned an' Forsaked on it, but maybe I could do another one specifically on the earlier versions of the story? I dunno).
So now Roy Thomas, Neal Adams and Tom Palmer take over the series and they quickly write off the Mesmero/Magneto team-up in X-Men #58 by having it be revealed that Magneto was a robot...
Now, do note that it sure seems to be implied that the bad guy thinks that Magneto left a robot in his place, Doom-bot style, right? Perhaps that is why it was never resolved just WHO built the robot back then, as maybe Thomas and Adams didn't think it WAS a mystery.
However, when the real Magneto encountered Mesmero in X-Men #112 (by Chris Claremont, John Byrne and Terry Austin), he noted that they had never met before...
The answer would not be revealed for over a decade after the discovery that Magneto was a robot!