From the website:
Protect your British and Polish armies from aircraft attack with the new Crusader AA Mk II/III.
After the completion of the North African Campaign, the availability of better tanks such as the Sherman and Cromwell relegated the Crusader to secondary duties such as anti-aircraft mounts or gun tractors. In these roles it served for the remainder of the war.
The Anti-Aircraft versions of the Crusader were developed in response to the perceived need to counter German “tank-busting” ground attack aircraft. They consisted of a Crusader armed with twin Oerlikon 20mm guns for anti-aircraft use.
Mk III only differed from the Mk II by the position of the radio, which was moved to the hull in order to free some space inside the turret.