These holdfasts are made by Gramercy in the USA. They are made using a modern formed wire process. A bit like spring steel and I would say virtually unbreakable under normal conditions.
The holdfast is used in a 3/4" hole in your workbench top. A 1 3/4" thick or thicker top is required for them to function properly. If your top is thinner you simply add a packer under the bench where the hold is to go. They have a 6 1/2" reach and the maximum clamping hieght is 7 1/4" in a 2" thick benchtop.
Imported into Australia by Colen Clenton.