I seem to remember a time when stereo records were incompatible with
mono cartriges, which lacked the vertical compliance necessary to
avopid damage to the grooves. Sometime in the late 60's or early 70's,
howvere, I remember "compatible stereo" records being introduced. About
this time, the appearance of the records also seemed to chamge. Before
this, stereo discs looked quite different. I remember having Peter Paul
& Mary records in both mono and stereo, and the stereo ones looked
quite different. The grooves appeared 'deeper'.