Interesting, do you have a reference site I could look at them, I can’t find anything on “slate shingle anchors”It is an anchoring device used to hold slate shingles on the roof.
Maybe you have other names for them in the US, but over here they're generally called 'tail rivets'. There's some generic information here:
copper tail rivet fixings 2
'Eternit' is a trademarked name for fibre-reinforced cement used to make cladding and roofing sheets and tiles among other things. The trademark has also been extended to cover bituminous products in roll form and various other roofing-related materials and fixings.
https://en.wikipedia.org/wiki/Eternit