Many of the eBay magazines are the late style with removable floor plates. Most do not have a Rampant Colt stamped on the floor plate. There is some controversy as to the maker of such magazines. Depending on the vintage of your Service Model Ace, you will need either a fixed floor plate version, or a removable floor plate version with the Rampant Colt stamp.
Functionally, there is no difference (unless one counts the "soft" pot metal followers in later magazines, where the "shelf" that operates the slide stop wears off more easily), but for "collector" purposes, the proper style should be selected to fit the vintage of the pistol. (On second thought, I guess all Post-War magazines have pot metal followers, so that is not a factor.)I didn't think there was any practical difference between a "fixed plate" mag and a stamped removable one, as long as they were for the "Service Model Ace" and not the older "Ace" model.