Can you tell me what this thing is?

[YamahaFreak]

I found it at Orlando HamCation this past Valentine's Day weekend on someone's free table, after they'd packed up and left. I have done as much research as I can with what I have, and I have come to the conclusion that it is the transmitter section of a General Electric mobile radio repeater, perhaps a MASTR series unit? I grabbed it with the hope of cleaning it up and using it for a display/visual interest piece, what with its visible tubes and other parts. I cleaned it and replaced broken and missing tubes. (Ignore the 6SN7GT; it's there to hide an empty non-tube socket, for display.) The large one at right is a 5894 twin tetrode; the rest are fairly garden variety receiving tubes. The unit bears the model or part number 4ET48A11, which Google turns up virtually nothing about. I'm hoping that someone, somewhere, will know all about this thing and can tell me how to apply power to it and light it up. I assume it is a 12 volt DC unit, given the tube heater voltages and the absence of a transformer. There is one lead coming out of the chassis (hidden in pic) with one black, one red, and one uninsulated (ground?) wire, which I assume is the power inlet. Measuring across the black and red wires with my multimeter shows open - perhaps a switch on an external control unit must be closed to power it up.