flatpacks do have incredible sound. You could tell the quality of the
carts was very good in the 1960's- other than the gooey roller ordeal

the later Columbia House carts are very good too- with superior pads
and great sound

RCA Music Service/Club carts of the 1980's, verily suck ! I have a
Hendrix Smash Hits that is basically unlistenable, yet a mint label.
Easy to tell it was dubbed at 2x speed or higher.