 Complete the following phrase:  As the initial contract was for 300 vehicles, General Motors Fisher Tank Arsenal produced the last 300 M10A1 tank destroyers in January 1944 without turrets for immediate conversion to M36s. This conversion lasted from April to July 1944. The contract was later increased to 500 vehicles, as it was decided that existing M10A1s were also to be converted to M36s. The requirement was later increased to 600 vehicles on May 15, 1944. As it was found that the M10 tank destroyer had struggled against German tanks like the Panther and especially the Tiger during the 