A low-impedance/high-impedance microphone matching transformer:
The transformer is a Shure Brothers A86A.
Case diameter 1-5/8", length 2-7/8", 7 foot cable. Shipping weight 1-1/2 pounds.
Model A86A. Code: RUDEB. List Price $13.10"
(Link to a Shure catalog listing it at Preservation Sound.)
That was pretty a high price, back when this was new. Frequency response is cited as "within 1 dB, 20 to 20,000 cps," which is darned good. The various low impedances are selected by removing three screws on the low-Z side, opening the case up and moving a jumper.
Does this one work? I don't know yet. It was priced to move at $3.00 and for that kind of money, worth finding out.