The tamper is the easy half of the Vitamix fitment problem, because unlike the blade, Vitamix will sell you the right one, and two of the best-reviewed options on the shelf are its own. The catch is that a tamper is cut to a single container length, and the maker is blunt about why it matters: Vitamix states that "lids and tampers are not interchangeable between different container styles, types and sizes", and an owner puts the reason plainly, a mismatched tamper "might reach the blades."
So the fit is the same split as everywhere in this cluster. The classic tamper is for the tall 64 oz Classic container on the C-Series machines. The 11 in low-profile tamper is for the short 64 oz and 40 oz containers. A classic tamper in a low-profile container is too long; a low-profile tamper in a tall container cannot reach. Match the tamper to the container, not to the machine on the box, and if you own an Ascent, the longer S2 tamper is the one that spans its containers.