subk - yabbut a quick google also shows an amendment to the fisheries act, one to the Fair Campaign Practices Act, and the Jumpstart our Businesses Act all either called Amendment 15 or with 15 amendments to the acts. Almost all the references to the US Constitution refer to the 15th Amendment, not Amendment 15 (or XV). So I think there's still room for curiosity.
Though after all that it dawns on me you may actually know the horse or the story behind it. If so, thanks.