Around 1991. There are switches with both old and new sort of parts as well. For example, I have several sets with the older type of springs and logos, but modern stems. The old type of stem doesn't have a "tooth" in the crossbar. It's the stem that causes the difference in sound (and perhaps a tad bit sharper tactility, but I haven't tried with sound isolation, thus I could be biased); I haven't observed any other differences in feel (esp. smoothness).