Documentation at dev-docs.org (field service manuals, user guide)
5¼ inch single density, single sided, 90KB capacity floppy drive.
Check out other members of the family: Atari 800 and Atari 410 Tape Recorder.
Power Supply Board
A CPU board’s screen fragment with a cushion tightening the CPU:
Micro Peripherals Inc. (MPI) Model 51M