A peg, pin, nail or dowell (inserted in a hole to lock, hold s.t. in place, connect two thing together, etc.).
To insert a peg, pin, nail, dowell, etc. in a hole (in order to lock s.t., hold s.t. in place, connect two things together, etc.).
The -an imp. focuses on the peg or pin to be inserted. The -un imp. focuses on the object that is having s.t. inserted into it.
Who inserted a nail in that window (to lock it).