Quoted from pencilneck:
If it hits a T-nut, then tap the T-nut out and just use a washer and nylon nut on the bottom side, easy and done.
Yes, this. Mine does not have a T-nut, it has the washer and nylock nut, and it came that way from the factory. I suspect someone added the T-nut on yours, its not supposed to be there. My Earthshaker however, does have the hex base post and T-nut style post... The only time Williams used the hex-base post and T-nut style, is if there's an assembly under the PF that mounts directly under that post (usually a flipper assy), and Whirlwind does not.
The post you bought is the correct one, its not supposed to have the hex-base, but it cant be used with a T-nut. Alternately, if you use the post WITH a hex at the base, you HAVE to use a T-nut.
The shaft of the post (you bought) with the round base is longer, so that it protrudes far enough through the PF, so you can get the washer & nut on the back.