The French had their own size & threading standards, which Peugeot used on their bikes 'til the early 1980s. The French spec was 28.0 mm, but more common 28.6 mm derailleurs typically fit well enough. And if a 28.6 mm derailleur clamp doesn't fit quite right, you can make a shim from an aluminum can. (It's easy to trim to size and shape so it'll be completely invisible.)