[bestgenman]

Larry, I looks like a sensor made by NASON which I use on gensets.  I t is for oil pressure and by the terminal arrangement on top I'm guessing is is a switch rather than a sensor ( such as oil pressure which would be a pressure driven variable resistance).  It could be used in a number of applications and the switch's pressure setting determines when the contacts change position.  Most of the switches used on gensets operate at 12psi falling, although  that varies.  The three terminals indicate a "form C" switch which is a common- normally closed- normally open configuration.  I use Davidson sales for the NASON product.  email me if you want their phone number.