This is a Hewlett Packard ( HP ) / Agilent 85021B Directional Bridge
Manufacturer Description:
The Agilent HP 85021B Directional Bridge is used with the Agilent 8755, 8756 and 8757 series of Network Analyzers.
It has a frequency range of 10 MHz to 26.5 GHz. The 85021B has a APC-3.5(f)) test port with directivity of >40 dB from 40 MHz to 20 GHz and >36 dB to 26.5 GHz. The rf input connector is type N(f).