A truly Rare & Wonderful 70's thing, infamous  Roland Japan AP-7 Jet Phaser, a unique blend of analog phase & analog phase with fuzz.
The Jet Level control is NOT a volume control, more an intensity control . It does seem to have a significant jump at about 2 o'clock.
In great shape for age, with the face-plate lift that was also evident on the 1 other of these I had a few years ago.

All switches , controls & pots in working order when tested before listing, but please bear in mind, this is 50 odd year old piece of gear that has spent it's life being stepped on, so it may have the odd snap, crackle or pop when engaged
from effectsdatabase;

Jet Level - controls the amount of distortion

Mode  - (Jet 1-4/Phase 1-2)?

Resonance - sets the amount of effected signal fed back into the effect (for a stronger effect)

Slow Rate - controls the speed/rate of the LFO wave that controls the effect

Slow/Fast - switches between the speed set by Slow Rate and the maximum speed. When switching from one to the other the speed ramps up (or slows down) smoothly, similar to a rotating speaker.