In message , vey
writes
It is a hall effect magnetic proximity sensor.
Here is the data sheet:
http://www.datasheetarchive.com/preview/3528026.html

I tried putting an ohm meter on it, but a moving a magnet around it
doesn't make it change.
I doubt it, the first page block diagram shows it has an internal
amplifier which would need power to function.
--
Clint Sharp