The first two photos are of the B car emitter and at some point in time someone sanded and polished the exterior smooth to remove the rough casting finish including the words cast into it. That third photo is of the A car emitter and you can see how the word Receive is barely visible (mostly sanded flush). The word Tone is hidden below the belt material.

It looks like there may have been some type of switch mounted to the exterior that pivoted on the round boss. It doesn't look like the switching mechanism had a whole lot to do with the actual function of the part itself though since the inner bores do not connect to it in any way.