It's likely part of some RF transceiver, likely military yes, or aerospace stuff.
There is an input marked "NRZ", which I could guess is a modulation input (NRZ modulation), so it's likely made to transmit data. As to finding a manual for this, good luck.
And yes, Beryllium was commonly used for insulation purposes in semiconductor packages, especially in RF stuff. I think it has largely been replaced by Alumina. Beryllium is very toxic.