Part ID Help

I was wondering if somebody might be able to help me.

I have had the part below in my collection for quite a while,
but I am not sure what it is. I was told it came from a Hampden
so I am not sure if that might help with the identification.

It seems to be brass, and is about 3 1/2″ across, with the markings VOLATILE AC, NORMAL AB, and OFF.

Any help would be great.

Johnnie