By definition, only the 1st Variation has an ATP (Army Test Proof) and it is located next to the Hammer on the left side of the Frame. I believe what you think is an ATP on the Slide is a typical Eagle/N Proof Mark - there are three, one each on the Frame, Slide, and Barrel. Also, the last ATP marked HSc was serial number 715323 so your new acquisition is well beyond that number.