I picked this up in a heap of old computer, radio, and telephone equipment, from a guy who had been an engineer in the Navy, then an engineer/lineman/programmer at Bell Labs in NJ:
It's an *actual* milliwatt! I can't find the BSP for it online, but I did find a hardcopy on eBay, so I'll scan that in when I get it.
Battery test points, this is the battery (well, the top of an old one):
45V "B batteries" were common in old radios and other higher-than-we're-used-to voltages were common in other types of test gear. For instance, a kick meter uses a different 45V battery (looks like a giant 9V and is still made).
Top of the internal circuit subassembly, the battery goes in the space seen at the top of the picture:
Here's the circuit:
Typical Western Electric, potted networks, switchboard jacks, and expensive resistors and capacitors. Not yet sure if the pot varies pitch or level. You can see there's a single very old GE transistor in a metal can package clipped to the side of the uppermost (4002A) network, presumably the only active component in the circuit.