Several types of M24640 come in armored variants, as marked with an “A” added to the lettering of the category. For example, Mil-C-24640/1 is in the non-watertight, non-flexing category called DX, with an armored variant DXA. Because the purpose of the M24640 is added safety, asbestos and mercury cannot be used in manufacturing as specified in the Mil-Spec standard.