Sennheiser EM100 G3 wireless receiver. Very popular EW100 G3 platform for serious musicians and sound companies.

This is the receiver only, as shown, just add whatever transmitter you want (microphone, beltpack, etc). It's in very good shape overall, just a little casing warble on the right edge (just cosmetic - see pictures). 

It does not come with a power supply or antennas.

A-Band - 516-558 MHz - Be sure your transmitter is on the same range.