Drop target shoulder stud. Part #5458-26, I believe. There are two sizes, this would be the smaller of the two if I'm not mistaken...the larger one is for in-line drop targets.
I got a couple from Pinball Resource a few years back. They are not listed on their website, but they were able to provide used pieces with the above part number, game title and descripton of part. Let them know you'll need the e-clip as well and they'll send the proper size.