large old brass Airguide barometer, vintage barometer looks rough but works

Vintage Airguide barometer, 5" in diameter.

The brass housing is very rough and corroded as you can see in the photos - though strangely, the barometer seems to be working fine! We've been watching it over the past couple of days.

The gauge dial is under a convex cover glass, without noticeable problems to the face or the glass.