ive been speaking to my local boffins and theyre also baffled, it has a 25.4 seat tube diameter suggesting cheapo, but butted tubes suggesting 531 front end. the chainstays length i have no idea about i was astonished when i built it and compared it with other road frames of a similar era, i still havent grabbed the frame number which im sure should help.
To be honest it rides pretty well, though obviousely without the 531 forks shown above, cheapo tange chromoly forks from my fixie will have to do till some straightblade or 531 forks can be dug out.