I ran into this exact same problem last weekend while tearing into my Pinbot, and I almost asked a question here about it. Weird!

Mine has been snapped off below the playfield. I'm pretty sure it's a threaded post because I think I seen a T-nut half buried underneath the moving target assembly. Digging around the manual, IPDB and the usual pinball sellers, I found myself coming to the conclusion that the post must be the same as the one in the center dividing the left and right lock with the yellow post sleeve. If I remember correctly, that's the only one in the manual that has a part number tied to it. I think it is a 02-4036 -1 or -2 but I'm not 100% certain yet. Does anyone else know for sure?