This is a Bird No. 43 Thruline Wattmeter. It is used to measure the output power of transmitters. The instrument has a directional coupler that is inserted in the transmission line between a transmitter and the load (either a dummy load or an antenna). The power range frequency range is dependent on the sampling element (or "slug") inserted in the front of the meter. I have a few slugs for VHF. The slugs are expensive.