Actually, a troop in WW2 was originally of 3 tanks - a single Firefly was added to each troop of Sherman equipped armoured regiments in the European Theatre in order to "up" the firepower making four tanks per troop in total. Each Squadron consisted of 4 troops plus an HQ troop of 3 or 4 tanks. There were 3 squadrons in each armoured regiment. The 4 tanks in each troop had the callsign alpha through to delta, with the firefly having the callsign "Charlie". The typical formation for the troop was a triangle of standard gun Shermans with the firefly a little to the rear - any contact would usually result in the call to "bring up the Charlie tank".
Armoured regiments equipped with Cromwells, rather than Shermans, were sometimes issued with "Challenger" A30 tanks rather than Fireflies to give them their 17 Pdr upgrade. Independent Armoured Brigades (usually equipped with Churchills and operating with an organic infantry division) did not receive a 17 Pdr vehicle, and maintained the 3 tank troop.
Last edited by Wensleydale; 7th Jul 2015 at 12:46.