Re: MiniDisk? Bah!
You can still get 78 styluses for several well known cartridges. I was surprised when I found that Ortofon still had a 78 stylus for many of their removable stylus cartridges in their catalogue.
Many BSR decks that used to be in 60's and 70's record players (like the popular Dansette) and phonograms had a dual purpose stylus with an LP stylus on one side, and a 78 stylus on the other. You could rotate it to select the one you wanted to use, and there was a plastic tab attached to help you rotate it, and display which was currently in use.
78s encode the sound vertically. Mono LPs encode the sound horizontally, and stereo LPs had the left and right channels encoded separately, one on each side of the grove, with the walls of the grove at 90 degrees from each other, 45 degrees from the verticle.
Playing a 78 disk on a stereo cartridge generally means poor quality sound, where the wider profile of the grove means that the narrower stereo stylus drags long the bottom of the groove, where the dirt and dust accumulates, whereas on a stereo LP, it sits touching the edges of the much narrower groove, and does not touch the bottom. Also, for a low arm mass LP turntable, the whole arm would go up and down, rather than the stylus moving relative to the body of the cartridge.