Posts come in 2 sizes... Standard (3/16") and Heavy (5/16").
This one is the standard size and uses a 3/32" Allen key. It screws into wood insert PN# 9476. Depending on your needs, you may need the heavy machine post instead. FYI:. For a short time, Phillips headed, bubble necked posts were made. This would be the correct replacement for that type of post.