I think there are two versions but probably just die changes.

In July of 1942 the Seymour Products Company was awarded a government contract to manufacture 4,882,043 Thompson thirty-round box magazines, purchase order number PO# 1939. The first magazines from Seymour were accepted by the Springfield Ordnance District in September of 1942. By December of 1942 production had reached 700,000 magazines per month. The announcement of the M3 submachine gun adoption, and subsequent cancellation of both the Marlin Firearms Company U.S. M2 Submachine gun and the Thompson Submachine Gun, production contracts for the thirty-round magazines were canceled on February 12, 1943. Because of materials on hand and quantities of magazines in process, production of the magazines was allowed to continue on a reduced scale until April 30,1943.

Because of delays in the production of the newly adopted M3 submachine gun, manufacture of the Thompson Submachine Gun had to be extended an additional seven months. Because of the substantial amount of additional M1A1 Thompsons beings manufactured more magazines were needed, and Seymour's contract for the production of the thirty-round magazines was reestablished. On September 1, 1943 the Springfield Ordnance District awarded a new contract to the Seymour Company. The new contract was for 2,070,000 Thompson thirty-round magazines. Supplemental agreements increased the total of thirty-round magazines manufactured by Seymour to 10,430,000.

In 1945 Seymour was awarded another contract, W-19-059-ORD-2693, to manufacture an additional 1,945,608 thirty-round Thompson Submachine Gun magazines under revised drawing D-35506, Revision 1-20-45. No government free issue involved. This brought the WWII production of Seymour thirty round magazines up to an incredible total of 12,375,608 pieces, making Seymour the largest producer of the thirty-round Thompson magazines during WWII.

BOTH ARE RIGHT HAND MARKED MAGS.