This is a Honeywell / Marion Instruments
Deviation Meter - Model MM2

Meter Face: 2.75 in wide x 2.75 in tall
Panel thickness: 0.35 in
Body depth (behind panel): 1.00 in
Mounting stud length: 0.5 in
Overall depth with terminal posts: 1.50 in

This meter was removed from operating equipment to be replaced by a digital version. This is a working meter.