The original (first-generation) .40 Auto magazine tubes look very much like the 9mm versions. The second-generation .40 Auto magazine tubes add an inward-protruding rectangular dimple on the left side that serves to prevent rounds in the magazine from inadvertently actuating the slide stop lever. The first-generation magazines use black or orange followers, while the second-generation magazines must use the medium blue or light blue (the latest) followers (see FAQ III-8).
The following photos illustrate the two generations of .40 Auto P99 magazine tubes, with the first-generation on the left, and the second generation (with the inward-protruding dimple) on the right):
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