On the museum website it is written that the secondery weapon is 20mm auto canon not 7.5mm machinegun “Next to the gun was a 20 mm coaxial machine gun. This was fitted to save using the main gun against lightly armored targets.” and it had 0.50 cal on the loader’s cupola
source:
AMX-50 Surbaissé: Learn About The Mighty French Autoloader (tankhistoria.com)