John, your magazine should have a SE/63 for an acceptance mark. Now comes the fun!! The 1937, Code S/42 was also made in the T-Block. To differentiate, the "SE/63" comes in 3 sizes. The small size is for the 1937 and the medium and large size is for the 1939. Unless one is familiar with these differences, it is difficult to immediately tell by inspection. I'll get some magazines out and make some measurements!
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