I picked up a small lot of things, and in it is is one of the above units in its original tin.

This appears to be for the RCA Type 4-A-1 condenser microphone.

I cannot seem to post photos on ARF, but the unit I have looks identical to the one at the bottom of this web page:

http://uv201.com/Microphone_Pages/rca_condenser.htm

The condition looks quite nice, but I have no way to test.

Any idea of value?

IF you can establish the mic element is good, and the condition is good, a microphone collector might pay a bunch for it. Might. If it is broken, or damaged, I would say then it is collectible but not nearly so much. I have no idea if they can be rebuilt or not but I would assume some of them might be repairable with the correct materials only.