GR 1710 ?
 

I just got a General Radio 1710-P2 Transmission/Reflection Bridge with 4
ports. On the front, RF IN, REFLECTION OUT, REFERENCE OUT (-12dB), and on
the left side, UNKNOWN B. I can't find any docs on google so can anyone
please tell me the frequency range, impedance and how the 4th port is used,
or perhaps where to find more info.
tnx
hank wd5jfr