What is this tape/pad attached to the heatsink pipes

https://photos.app.goo.gl/rWKSEXwNhmG7Zg...

I managed to lose a piece of it so needing to find a replacement.

It's paper-thin, silvery on both sides, fairly stiff and strong. Not sure why its there

but I'm guessing insulator/smoother. I was thinking of trying Nashua

hvac foil tape, which is rated to 125C.