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.htmThe condition looks quite nice, but I have no way to test.
Any idea of value?