Marienwagen II with AT gun

I have seen this picture of an ww1 German Marienwagen II halftrack with an AT gun.

Different sites give different mention of the gun, be it 5,2 cm Sk L/55 or 5,7 cm Hotchkiss L/40 but none of them actually fit the gun.
It has an very visibly and smooth tapering barrle, overall a small mesurement, and appears to have a small recoil cylinder under the breech. Its caliber appears to be something between 37 to 57 mm does anyone know more?