mountings of the 6-pounder, 17-pounder, 3-inch 20cwt guns and the 25-pounder field gun/howitzer on the Matilda II, Valentine, Crusader and Cavalier (Cruiser Mark VII) tank chassis. In October 1942 it was decided to progress using the Valentine chassis with a 17-pdr (which would become Archer) and 25-pdr (which entered service as Bishop). While there was a general move to a general purpose gun that was usable against both tanks and in supporting infantry, there was a need to put the 17 pdr into a tank for use against the enemy's heavy tanks. The Cruiser Mk VIII Challenger was a project to bring a 17 pdr tank into use to support the Cromwell cruiser tank. Delays led to it being outnumbered in use by the Sherman Firefly—but a derivative of Challenger was the more or less open-topped variant Avenger, which was delayed until post war before entering service. A cut-down 17 pdr, the 77mmHV was used to equip the Comet tank in the last year of the war. The closest the British came to developing an armoured tank destroyer in the vein of the German Jagdpanzers or Soviet ISU series was the Churchill 3-inch Gun Carrier—a Churchill tank chassis with a boxy superstructure in place of the turret and mounting a 3-inch anti-aircraft gun. Although a number were ordered and fifty delivered in 1942, they were not put into service as the immediate threat passed. The design was rejected in favor of developing a 17 pounder armed Cromwell tank variant, ultimately leading to the Comet tank. The Tortoise "heavy assault tank", intended for use in breaking through fixed defensive lines, was well armoured and had a very powerful 32-pounder (94 mm) gun, but did not reach service use. By 1944, a number of the Shermans in British use were being converted to Sherman Fireflies by adding the QF 17 pounder gun. Initially this gave each troop (platoon) of Shermans one powerfully armed tank. By war's end—through the production of more Fireflies and the replacement of Shermans by British tanks—about 50% of Shermans in British service were Fireflies. The Sherman Firefly, however, is not considered a tank destroyer since it could still perform the other duties of the regular M4 Sherman, albeit the Firefly was less capable due to the late development of a HE round for the QF 17 pounder. Romania Until 1942, the Romanian tank force was equipped exclusively with