The hull is clearly an early leopard prototype, and the gun as pointed out is an L7. At first I was thinking some kind of jagdpanzer or flakpanzer prototype. However, I can't see a way for the gun to elevate, and the suspension doesn't look modified to allow it kneel like an S-tank (hard to tell just by looking of course). The turret is odd too - flat topped with large hatches, and there looks to be reinforcing struts to the rear. My guess is that it isn't a vehicle prototype but rather is a weapon test platform, in this case for the L7 gun. The 30 mm might act as a spotting rifle for the gun similar to the 105mm recoilless rifle. Other than that, I have no clue as to what it would be for.