Rear section of right-hand* flip-up window frame, incorporating the long-type peg (which locates in the rubber grommet on the older cars, to hold the window up).
A very rare new old stock original Citroen part, which will need preparing & painting before fitting.
*”Right-hand” is as you sit in the car, looking forwards, (as in left-hand drive or right-hand drive / conduite à gauche ou conduite à droite).