Not Cinelli--wrong seat cluster, wrong fork bend, and no stiffening tangs on the fork. And not DeRosa--wrong everything, except, of course, the cutouts.

The fastback seat cluster, "stock" Reynolds fork bend, domed fork and stay ends, and fine workmanship suggest a small US or UK builder. The work is too nice but idiosyncratic for it to be French--the best French bikes were very conventional-looking. The convergence of vertical dropouts, brazed on cable guides, bottle and lever bosses, and lugged construction suggest it was built in the 1980s. I've seen this style of work on Witcomb (US and UK), Roberts, Condor, and Eisentraut. Not to imply that this frame was built by one of these, but these builders were influential or representative of a style.

If it had an R cutout in the bottom bracket, the seller wouldn't be entertaining the idea it might be a DeRosa. And yes, the screw holes on the seat stays are curious. Looks like a nice frame