
(Firefly IC WIP)
Hopefully this photo above will help. While a .50 cal was mounted, it was far more common for the Commonwealth Shermans to use a .30 cal.
Here's the cut out in the turret rear of the Tasca turret as pointed out by Tom. And as Tom says, it was part of the factory conversion when building these versions.

You may also wish to note that not every Firefly had a stowage box added to the armoured radio box.
Gary