Don Pearce wrote ...
I was interested in how heavily modulated vinyl could be, so I popped
an old record (Long Hot Summer Night, Jimi Hendrix, Track Records
1968) under the microscope for a look. And what did I find? Two
adjacent grooves clearly broken into each other:

http://81.174.169.10/odds/grooves.jpg

Was this a really common back then, or is this kind of thing a rarity?

Back in the 1970's Shure sent out marketing people to local stereo
shops demonstrating how good their latest phono cartridge (V15?)
was. The show was great until I brought them my "killer album",
(IIRC Neil Diamond "Taproot Manuscript") which had an extreme
guitar transient in the intro to one of the songs. They never found a
solution to tracking that song. My Stanton at home came closer
to success than their pickup.