I just purchased a vintage Peugeot 501 road bike. Can someone tell me where the seat height adjustment is?

As a fairly logical person I may be able to help you deduce the intricacies of your seat height adjustment if provided with a picture of said assembly, sir.

Most bikes have a binder bolt or collar at the top of the seat tube. Loosen that to adjust the extension of the seat post holding the saddle. Some French bikes may have a quill type seat post with a bolt at the top of the post, under the saddle to accomplish the same thing.

This is possibly the coolest factoid I've learned all week.

Edit: Found a video showing one...