During 1942-43 the standard size of an armoured troop (platoon) was three tanks and had served the British well. However, it became clearer as the war progressed that the standard 75mm gun of the Sherman would not be powerful enough to deal with the ever increasing amounts of armour the German were applying to their tanks.
Already in service in 1943 was the excellent 17 pdr Anti-tank gun and it was decided to mount this in a Sherman turret. After some trial and error, a mounting was affected.
The new 17 pdr armed Sherman was dubbed the Firefly. The Firefly was then issued to the armoured divisions, with most formations issuing them to the Armoured Troops as an extra tank, thus bringing the strength of a troop up to three Shermans and a Firefly.