The Tektronix P80 is the probe for the Type 80 plug-in for the 581 and 585 oscilloscopes. It is a cathode-follower probe, and contains a 6688 pentode. The tube functions as a phase splitter, not a simple cathode-follower.
Vents for tube
Amphenol 165-14 connector, plugs into Type 80
Schematic