Generally, a picture is worth a thousand words and when seeking advice on the forum, it is wise to include good clear pictures if possible.

As for your seat post, I have never seen such a set-up on an early eighties Peugeot, hence even more reason for me to want to see a picture. Also, if you measure the seat post diameter, I just might have exactly what you need in the form of a seat post that works much like a steering stem with a pull up wedge at the bottom of the post. However...

If your Peugeot has thin chrome moly tubing, be careful with this kind of seat post. The wedge action could expand the seat tube if you are not careful.