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E.g. https://uk.farnell.com/3m/120mm-1m-0-5m ... dp/2820143 where you will find a data sheet.
Those thermal pads were standard on the GigaTech PAs and blown heatsinks. Mine was in a bit of a state so I scraped it off and used heatsink compound.
The "Polish PAs" that Noel and I now use also have one. Seems quite effective when properly mounted (>200 W dissapation on mine)
Not seen an image but back in the day 2N3055 devices in a TO3 package had to be insulayed from the heatsink but still im thermal contact using thermal paste
Mica washers were used and other materials.