Picked up a 100+ year old shop book at a sale. One of the illustrations was a diamond toolholder. Very much the same as the modern one that is advertised (or used to be, seems to be gone now). Showed a square cutter held at an angle to form a diamond face, ground flat.
Angle was such as to allow the cutter to be a 60 deg threading cutter.
I had thought these were relatively new...... But there it was in a 100 year old book, represented as already a well-known item.