(For s.t.) to penetrate all the way; to reach to the bottom (as a post reaching the bottom of a hole).
The -an imp. seems to have a slight bit more focus on the location whereas the pa--un isn't as interested in the location. They can be used interchangeably.
If that board is two inches thick, you should use three inch nails for nailing it so that they will penetrate all the way through the board and reach that post.