To keep to the 'Thread' i.e. Comet.
According to B.T. White, British Tanks 1915 - 45
Battle experience indicated the need for a cruiser tank with a gun heavier than the 75 mm., so in July '43 Leyland Motors Ltd., then responsible for the production oCentaur and Cromwell tanks, began the design of a new 'heavy cruiser' to be armed with a new gun. This weapon known as the 77 mm., was a version of the 17 pr. with a shorter barrel and a somewhat lower muzzle velocity. The new tank, named Comet, was similar in layout to the Cromwell and had the same Meteor engine.
The first prototype was ready for test in Feb '44, and after several modifications to the design the production models began to be delivered from September of the same year.
The Comet was not used in action until after the Rhine crossing in early '45. It was very successful, as it proved to be fast and reliable and the 77 mm. gun was highly accurate.
It's all too easy to wonder why the design couldn't have started perhaps six-months earlier!?
However there was an alternative - the Challenger, 1 ton lighter, with 17 pr., gun, max speed 32 mph.
The Challenger was a modification of the Cromwell design with a longer hull and modified suspension, including an extra pair of road wheels. A larger turret, designed and developed by Stothert and Pitt ltd., was fitted, mounted with the 17 pr. gun and a co-axial 0.30 Browning machine gun. This type of tank was used in small numbers in the campaign in North West Europe to stiffen up the fire power of regiments equpped with British cruiser tanks.
The initial design of the Challenger was drawn up in 1942 by the Birmingham Railway Carriage and Wagon Compny, and a toatl of 200 tanks was produced under the parentage of this firm.
Unfortunately no details were given on the service date of the Challenger, with a design date of '42 dhould have been earlier than the Comet. The only other alternative that would be compatable with the time-frame would be the A.33 Heavy Assault Tank - but with a 17 pr., gun rather than the 75 mm!