In the late 60’s/early 70’s, record companies were deleting from their catalogs the mono versions of LP’s. All through the 60’s, LP’s were offered in both formats, the stereo version selling for a dollar more, mono going for around $3, stereo $4. Rock bands were using the new multi-track recorders to create stereo effects, particularly of the "psychedelic", back-and-forth, ping-pong type. This topic came up here a few months back, people mentioning the first two Hendrix albums and Moby Grape’s debut, which contain that exact thing.
Nobody wanted mono anymore, except for myself and people like me who had discovered that quite a few LP’s were not available in true stereo, the "stereo" version of certain LP’s in truth being electronically-created stereo simulations---a horrible invention. The record companies either punched a small hole in the corner of the LP cover or cut off the corner, and sent them to retail outlets like drug stores, who sold the LP’s for 59 cents apiece!
I had a group of stores I would go to a few times a week, looking for mono Kinks, Beach Boys, etc. LP’s. I would buy every copy of a mono pressing of a "good" album I found, ending up with multiple copies of some titles. Those LP’s were used to do trades, with someone who had found a mono copy of a title I hadn’t. When I discovered Goldmine Record Collectors magazine in 1976, I started selling those 59 cent mono LP’s for $20.
I still have those mono LP's, but only one copy of each title. The multiples I now have are LP/CD duplicates, with different LP pressings of some titles particularly important to me, or unusual in some way. Here's a good example: T Bone Burnett's Truth Decay (nice pun!) was released on Takoma Records, the label originally being distributed by Chrysalis Records. The last song on side one ("Driving Wheel", a very cool song done in Rockabilly style) contains a "false" ending---the song ends, the sound of the instruments fading into silence. After a long pause, the band starts up again, and plays through a long, slow fade-out. At some later date, the Takoma label was distributed by Allegiance Records, who remastered the LP. Whatever engineer did the job didn't realize "Driving Wheel" has a false ending, and cut off the song before the band resumes playing!