I picked one up at a jumble a year or two back at a 'bargain' price. Fitted with a Campag Record headset, Nervex Pro lugs, Zeus 2000 dropouts, a C&G finish with their trademark hand written script for the original owners name, it looked good. However, the headset was shot and made up of odd bits and when I checked the tubing up from the bottom bracket it was seamed - so nothing special after all, and it was heavy in comparison to other 531 frames I have. It did however have a 'proper' Q number under the BB, made up of the 'O' and a squiggle but it was in the 5000's denoting after 1980 so a Frank Clements one.
Caveat Emptor!