"(my 1966 mono LP sounds like a 3rd-generation copy made at 3-3/4ips)"
Maybe the "stamper" theory applies here?
There was the LA, Scranton, Jacksonville and Abbey(Jersey) pressing faculties.
I have one of the variants a #28
Sounds quite good, but haven't compared to others.
There are some comments about the pressings from the stamper freaks mentioning "this one sounds better that."
I have the 1st 5 all LA press mono. They sound fantastic.
All found over time bin diving SoCal and statewide.

