Was first featured in BASF's first chrome cassette in 1971, the LH series cassettes (SK in the American market) began to be equipped with a pair of additional parts that ensured smooth tape laying.

Later they sold the patent to AGFA. It wasn't before 1992 that you could find some BASF cassettes being technically the same as Agfa - by that time BASF had taken over Agfa's magnetic tape division. But these didn't feature SM any more.

For a long time all BASF cassettes incorporated a patented tape guide called the SM or Security Mechanism guide. This guide, of Deirin® plastic, has flanged edges which work somewhat like packing arms in aligning the tape from or onto the hubs rather than relying solely on slip sheets that contact the tape edges. All cassettes suffer some degree of tape edge damage after repeated passes, especially at fast winding speeds, because of the edge/shell contact. The SM guide is designed to control and guide the tape even in cases of serious damage so that the tape does not ride over the tape pack and get caught in the hub spindle area. The most rugged use a cassette is likely to undergo is that in car stereo systems. An endurance test conducted by a noted car stereo manufacturer showed fewer failures in cassettes using SM guides (BASF cassettes and others) than in those without them. Tests conducted by Studer/Revox confirmed these findings. The SM guide, however, was not without its problems. Those cassette decks that wound or rewound tape at high speeds without slowing down could send a sudden reverse rotational jerk to a tape pack when the tape reached its end, and this reversal could through a loop in the tape or leader left on the take-up hub. This loop could get caught at the top of the SM tusk and cause a tape jam in the very device meant to preclude jams. For this reason BASF eliminated the Security Mechanism and relied on other methods to prevent jamming.