I'm reviving this thread because I went to install this derailleur on an unknown English bike from the 50s, and discovered the clamp is too small. The bike has a standard 28.6 seat tube that had a Campy fd in use but I wanted to use this one because of the built in cable stop. Apparently this one has a 27.2 clamp. It was removed from a Puch built Sears Ted Williams Sport Racer from the 60s that has a smaller diameter seat tube. Were there different clamp size versions back then? I find it hard to believe Campy made a special size for Sears bikes.