Hi Dave,
Applique armour was applied at the factory, to all models of the Sherman from the Summer of 1943. While at the same time other factories were set up, tasked with fitting applique to tanks already built. Typically the armour covered the ammunition racks, turret right cheek and the drivers hoods.
All Firefly MkIC and Hybrid MkIC tanks had the full applique fit. However from late 1943 some of these tanks were fitted with a new design turret, having a thickened casting at the right cheek, making the applique patch unnecessary. All high bustle turrets were of this later type.
About half the Firefly MkVC tanks converted had applique armour, including a batch of older ex- US Army tanks that were remanufactured in the US and converted in late 1944 in the UK. Generally speaking, tanks shipped to England prior to the Summer of 1943 did not have applique armour. Remember that it took 3 to 6 months for tanks built in the USA to reach England. When the Firefly program started in early 1944, most of the tanks used were the final models of the 75mm armed mid-production models. and had full applique.
The Firefly had new armoured ammo bins situated lower in the hull. So technically it did not need the extra armour. But any armour is good armour and no attempt was made to remove it! Note that tanks fitted with applique were de-luxe models with a host of new fittings. These included spotlight, periscope guards, hatch stays and counter springs, up-armoured nose (not M4A4) and barrel cleaning rods etc.