Plug
From TrigpointingUK
Plug
A plug is the removable centre of a spider. It is made of brass and consists of two parts. The whole plug can be removed to allow a theodolite to be accurately plumbed over the upper centre mark at the base of the pillar. Alternatively, just the centre portion can be removed, allowing a beacon to be mounted over the approximate location of the mark. Finally, the centre portion can be inverted, revealing a centrally drilled hole, with securing grub screws.
