1974 Brain Box saves the day.

Yup, thats a thermally conductive insulating pad. The TO-3 had two pins plus the case to make the 3rd. The 2 pin layout was for it to fit in a common at the time tube socket. Im not sure how they even utilized the case as the 3rd pin in that application as it looks as if the entire TO-3 case (collector) is insulated? Show us the nudes tomorrow!

EDIT>> looks like one screw may be used for collector to whatever is under the case...time will tell<<