Not that it never existed, but it wasn’t what it is in the game…
Just browsing around YT lately and came across this video that suggests that the FV4202 was largely built from non-armour steel.
Specifically the turret was cast steel at the front, and the sides and rear only 30mm rolled steel - not armour grade see 8.03 in the video. (note we do already have 1 vehicle that has only non-armour grade Steel - the VT1-2)
And the 20 pdr used bagged ammo - not the fixed ammo used in the service version.
also of course it was not a prototype Chieftain at all - it was a test bed for various things some of which were later used in the Chieftain, but that’s not all that important…
Much or perhaps all of the info is also at FV4202 - The Tank that Led to Chieftain -