In connection with the description of German influence units and circuits, it should be borne in mind that these units existed in a large number of slightly varying form. One of the most common variations was in the delay arming clock fitted, resulting in minor vari-ations in the delay arming cycle but having no effect on the basic operation of the unit. The following descriptions will present typical assemblies of all known influence units, and all of the variations. German influence firing devices (units) were designated by the Allies with a letter prefix and a mark number. The prefix indicated the type of firing influence or influences employed as: M-magnetic, A-acustic, AM-magnetic-acoustic, AP-acoustic-pressure and MP-magnetic-pressure. The mark number indicated the individual unit, the numbers being assigned in sequence of recovery, with little attention paid to the design of the particular unit. A detailed description of the operation of each unit circuit, to-gether with various unit accessories, follows in chronological order, using Germn designa-tions.