Starting in 1969, US acetates began being made by Bell Sound Studios in New York City.
Most Bell acetates consist of The Beatles' Let It Be album and various Lennon material.
The discs featuring Beatles' recordings all contain the standard released stereo mixes.
The Lennon discs contain released mixes, promos and maybe early mixes.
Many of these discs come from the collection of Sam Feldman.
Feldman was an engineer for Bell during this period.