I have a student who recently purchased a 1959 172.  It has an AIRNAV marker beacon unit.  At least I think it is a marker beacon.  It has three lights, one marked "OUTER," one marked "MIDDLE" and one with the marking worn off.  It also has a meter on the right side the has no markings on it.

None of the lights light when I fly over the outer or middle marker but the needle always moves to the middle of its range when it is powered up.

Does anyone know if it is actually a marker beacon receiver and what the meter is for. 

Thanks
Larry