This part is attached to the bottom side of the rail. The rail bolt comes up from underneath the slate and into the underrail plate. This is what holds your rail to the slate.
Dome slate washers are also recommended with this item. They are spring-loaded washers that help hold the rail tightly to the slate.
You will need 18 per table, 3 per rail.
1/8" x 1 3/4"
3/8" bolt for center hole