The Macintosh LC 520 and Performa 550 computers use the PlainTalk microphone. Both of these computers have built-in microphones, but also have external microphone jacks. To use the external microphone jack, you need a PlainTalk microphone.
The PlainTalk microphone and the older microphone share a 3.5mm phono plug, but the PlainTalk microphone has a longer plug (.75 inch vs. .5 inch). Consequently, the old microphone does not seat fully in the microphone socket of the PlainTalk microphone.