On February 6,1942 the Crosby Company of Buffalo, New York was given a contract to produce 1,803,258 thirty- round Thompson box magazines on contract #W-740-ORD-78 for an average cost of 28 cents each. This was the first of several contract for Crosby. Crosby ended up producing 5,415,922 thirty-round box magazines.

Not all was rosy for Crosby. In September of 1942 it was discovered that 35% of the magazines tested would not fit into the MI Thompsons. The remaining 65% would fit the MI only with 20 pounds of hand pressure to lock into the gun. It was determined that due to the Crosby magazines having a more pronounced flare at the front of the body and the MI's receiver magazine well being reduced in size, there was an interference fit. The Augusta Arsenal devised a simple solution and produced some hand operated tools to reform the feed lips on the magazine. This operation also required to grind .060 inch from the front of the magazine lips. Cost of the modification was 6 cents each.

The markings on the Crosby magazine often confuses many beginning collectors who believe the CCO marks represent Colt's. Colt did not produce any thirty-round magazines for the Thompson.

The Sparks-Withington Company was awarded a contract on June 2, 1942 and eventually finished the contracts by mid 1943. The contract was for 2,840,500 thirty-round magazines at a cost of 27 cents each.