Here's the seatpost clamp hardware on my Motobecane Mirage, which I bought in 1975. This takes a metric open-end, metric box-end, or an adjustable crescent wrench.
Hope this helps.

(EDIT: when you raise the seatpost, be sure to observe the "minimum insertion" marking on the post.)


Metric nut on the right side. As others have suggested, take the seatpost out and be sure it's lightly greased. Your bike may, like this bike, also have a shim insert which should also be lightly greased.