Curiouser and curioser. There is a black "washer" between tag and pcb pad.
Undoing a bolt this was a standard black nylon TO3 mounting insulated washer
but on the underside. So instead of bolt head, insulator washer, TO3 flange,
mica, heatsink, pcb it is bolt head, flange , mica, heatsink, pcb, pcb and
trace, TO3 insulator upside down, solder tag, star washer, nut. Flange is
same thickness as pcb and nylon insulator is only long enough to half
penetrate the pcb so what advantage ? or cock-up?