Milestones:Plaque mounting details
From ETHW
Milestone plaque mounting details
The milestone plaques come with six mounting holes in the back. There are four at the corners: 2.75 inches/6.5 cm in from the corners (as measured on the diagonal) or 1.95 inches/4.596cm in from the edges.
There are also two holes along the mildline (6 inches/15.24cm) of the plaque. They are 4 inches/10.16 cm in from the side edges (or 5 inches/12.7cm from the center) and 6 inches/15.24cm from the bottom and top edges. The plaques come with six threaded bolts ½ cm in diameter and 5 cm long.
Below is a rough diagram of the plaque as seen from the back.