Actually the 17000K may be keyed and 26000K splined which are not interchangeable at all.A lesson learned - Count the splines on your pump. At least for the QSM11 there are two pumps; the early pump has 14 splines (Sherwood 18000K) and the later pumps have 10 splines (Sherwood 29000K). You mentioned 17000K and 26000K which keyed me. I had a pump replaced so I have to carry both impellers as spares.
Tom