I remember it. I had it. Not sure what happened to it. Mostly what I remember is impossibly long legs and a big head (with a lot of sculpted "hair"...why? Chris Reeve's hair wasn't that long compared to comic book Superman).
Anyway, it was a Mego product and part of a three-figure set that also included Jor-El (comic book costume with "Brando"-like white hair) and Luthor (no resemblance to Hackman).
These show up frequently on eBay...try searching for "Superman 12-inch Mego" or a variation thereof. You may also come across a 12-inch Mego Superman NOT modeled after Chris Reeve. In my opinion, it's a much nicer figure, though if memory serves it was exclusively for the UK market at the time.
The guys over at the Mego Museum message boards would know more. Try
http://www.megomuseum.com