I have noticed that the "DX" coded ones don't seem to hold the end caps as well as the Older cassettes. I am not sure if it is thinner steel, a different alloy or perhaps just worn tooling. Of course the other difference is that the newer design has the lips flat at the point where the newer cameras have the film type window, the original design in the 1980 period had the film come out almost perpendicular to the cassette. I set aside my old ones when I switched to the Auto Focus SLR, as they use at least the speed setting on the DX codeing.
I have tried the labels DX that a UK dealer sells, and was underwhelmed. (First off I had to get a friend to buy them and send them to me as the dealer would not mail them to Canada, and second they were not easy to place, and they wrinkled far to easily.) So they are not the answer.

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