Really? I thought tubulars were always that size. The again, what do I know? It'll be interesting to hear what others say.

As for the OP, I'm pretty sure 700C has been around in France since the dinosaurs roamed the earth but if you mean when did they start appearing commonly under that designation on clincher rims on bikes in the United States, I think sometime in the early - mid eighties.

My '72 Bottecchia originally came with sew-ups but the original owner swapped them out for steel clincher rims from Dolomiti at some point (the tires were actually labeled 28x1 1/8 IIRC) but those were 700C. They at least looked very 70-ish so they were probably available then through the aftermarket. BTW, that tripped me up when I bought it - the owners wife was handling the sale and knew very little about the bike. I could see the "Dolomiti" labes on the rims in the picture, a common Italian brand, and the dry rot 28x1 1/8 Clement tires so I assumed they were original and the rims would have been alloy but they weren't. I swapped those steel rims for a nice set of alloy Rigida clinchers like the original owner should have done in the first place.