Maybe I'm quibbling but there is no cap on those stays. The seatstays themselves are mitered and those points shaped into the tubing itself, no separate cap. Those points are longer and more slender than most, but other than that it's a common-enough seatstay style, used by builders in every major bike-building country at least sometimes. Eisentraut, Jeff Lyon and others in the US, Ron Cooper in England, Pinarello and Bottecchia in Italy, Merckx and Libertas in Belgium. No French examples spring to mind at the moment... anyone?

Here's one by our own Fredo76 , I think this is a frame he made (correct me if that's wrong):


Harry Hall made the points so long that they wrap over and join on top, not an easy style to pull off:



All just variations on a theme, but you're right that this mystery bike example is distinctive. I haven't seen it done exactly like this ever before. Too bad about the rust, I think I would sand that down and hit it with at least some primer if it were mine, to stop the rust progressing any further on this very distinctive point.