Whats really unusual is the top picture is not the same idler arm as the bottom picture
The one in the top picture of the eBay listing (in the Mustang marked box) is not correct. Not sure what it is, but not original FoMoCo. The one in the lower picture in this listing (in the Fairlane/Torino box) is a correct FoMoCo item, but if it is really a Fairlane part I don't think it will work on a Mustang. If it is Mustang, not Fairlane, then it is a P/S arm assembly. And yes, M/S and P/S idler arm assemblies for Mustangs are definitely different.