Hi Gino,
I’m with Stikpusher on this one. Once the M4 was standardized, a fitting for the A.A. gun was attached to the cupola split ring, this taking place in the spring of '42. Once the 75mm gun turrets started receiving the all around vision cupola in mid/late '44, a folding pedestal mount was added towards the rear of the turret. Your description fits a late war tank.