Rear Mgs - it is up to the OOB designer to determine if they are usefully placed or not. As with some odd armoured car I added to Swiss "blue" OOB that had one in each corner - there I made the decision to give it 2 since probably 2 would bear in each direction.
Many Japanese tanks have their "co-ax" in the turret rear, as do some KV and IS series.
SP does not have any concept of "rear" armament - see the UK Archer tank destroyer.
In the great scheme of things, its neither here nor there quite frankly. It might get some rivet-counter's underwear in a twist should you include one, but who really cares about that?
!
Andy