Der Tektronix Type 1A5 ist ein Differential Amplifier. Das Instruction Manual ist im Web auf Bama erhältlich, da steht alles genauestens drin.

The Type 1A5 is a calibrated differential amplifier, useful for measuring high side signals or for rejecting AC common mode signals. The plug-in can be used also together with an P6046 differential probe (on the blue connector) for the smaller voltage ranges, read the manual. This unit has been full cleaned, some waterdrops remain still on the front-panel. It's not a  working plug-in, needs a repair.