 The LM 42U mount was the most common mount used with the 3.7 cm Flak M42U.The 3.7 cm Flak M42U was the marine version of the 3.7 cm Flak used by the Kriegsmarine on Type VII and Type IX U-boats.The submarine's keel was laid down in November 1942 by Blohm & Voss of Hamburg.