Peter, Have you actually seen a local precancel device of any sort? Please explain your view of the US method.
Here is visual evidence why I believe the dot in Wert's example is a nail head: a 2x5 handstamp precancel device showing 3 views. Note the curved metal plate held to the wood base with metal fasteners of some sort.
Lastly, two impressions from devices where the mounting nails/rivets show:
It is fairly easy to find examples of local US precancels showing extraneous circular marks in places on the plate which would be logical attachment points to the backing. And of course the 100-subject double line electro plates are flat rather than curved, but still consist of a thin metal plate mounted to a block of wood.