All the research I've done does say it's a rixon.
I think that you might be interested in this link . . .
http://www.bikeforums.net/showthread.php/555832-What-Tubing
Note, especially, the serial number - which appears to be
the same as on your bike !
On the face of it, someone has taken a track bike; changed the rear dropouts, added a rear brake bridge and shifter mounts on the downtube. And then, resprayed it; the bike looks quite unlike it did 2.5 years ago - when it was in Turkey !
IF it is 753, the rear dropouts will be wider than they should be, due to having gears, where previously there were none. 753 tubing should not be spread (it won't coldset - it'd spring back into place). I wonder if the frame is 531sl or 531pro.
It
will be a 'good' tubeset; frankly I'd keep it. It has an interesting history - Rixons are rare bikes and I've only ever seen one in the flesh.
I can't answer the 'mystery' of the frame number, but I'm certain it is the same bike. You mentioned that it belonged to a guy who bought a Lambo Countach; well on the link that I've given you, you'll see that his user name was '
Miura' - wich is also the name of a Lambo which came out before the Countach.
The story kind of 'fits'; apart from the alterations and the respray (why??). If it'd been left as it was, it be worth at least double imo - it certainly had a very high 'bling' value; the hipsters would love a bike like that.
Anyone else got a better explanation ? Was R131 put on all Rixon frames ?