I've seen pictures of various tanks in combat both with and without mantlet covers. On what occasions would they be installed?

Speaking of American tanks, before 1945 the covers were for shipping and storage and would not allow the guns to be fired. In 1945 the Shermans and Pershings started mounting covers that filled the gap between the fixed and moving parts of the mount. These would be in place whenever possible, but the mounts were available before the covers were. Immediately after the war full covers were used, and again, if available hey would always be in place because they kept dirt and junk out of the moving parts and turret interior.

I believe the M1 tanks have an internal cover. This is better suited to maintaining NBC protection by keeping an overpressue inside the tank.

KL