The Reibel Modele M31 was the variation of the French Chatellerault M24/29 light machine gun made for use in vehicles and fortifications. In accordance with that role, it lacked a buttstock or sights (these were integrated into the vehicle or fortress mounts), was fitted with a very heavy barrel for sustained fire, and fed from a massive 150-round drum magazine. These were often used in dual mounts, with one left-hand and one right-hand feed gun (note that the bolt is easily swapped for feed from either side).
After World War Two, the Swiss purchased a number of these guns form France, rechambered them for the 7.5x55mm Swiss cartridge, and fitted them in AMX tanks. This particular gun is one of those, and it is mounted on a Swiss LMG-25 tripod using an adapter designed for gunsmith test firing outside the tank.
Thanks to the Kessler auction company in Switzerland for letting me take a look at this piece!