SD:Crusader AA Mk.1

 is a British Anti-air unit.

Overview
The Crusader tank was one of the most important cruiser tanks of the British Empire in the early years of World War II. Designed by Nuffield Mechanizations and Aero Ltd between 1939 and 1940, in response to General Staff specification A13, the crusader was a cruiser tank based on the Christie helical spring suspension, sacrificing armor for mobility and maneuverability. The small dimensions of the tank also reduced the size of the turret ring, resulting in a distinctive, angled turret designed to maximize its size and capacity. Although struggling in the hot climate of North Africa, the principal theatre in which it was employed, the Crusader was nonetheless instrumental in Allied victories in the early part of the Desert War, before it was replaced with superior American and British designs.

The Mark III was the most advanced version of the Crusader, upgunned with a QF 6-pounder (57mm gun), better armor, and a Mark IV Liberty engine, fixing many of the reliability issues found in previous versions of the tank. The auxiliary turret was removed, replaced with more plating and additional ammunition stowage, while the crew was reduced to just three by having the commander also load the main gun. The Mark I AA conversion replaced the entire turret with an open-topped superstructure featuring a Bofors 40mm anti-aircraft gun in a powered mount with an autoloader.