Good question Motor so here is your answers

closed cooling NON CAT and CAT motors - Mercruiser 170 degree t-stat part # is - 864493 these are side mount using a different t-stat housing - the t-stat mounts in the port side of the housing.

next is the 8.2 CATALYST engine also listed MERCRUISER 8.2 EC - 170 degree T-stat part # is 8M0045990

Next is the Mercury Racing engines - the QC4V Quads 1100 & 1350 & the Racing 1650 all use a 185 degree T-stat - SORRY I WOULD HAVE TO LOOK THAT PART # UP - as I do not carry them. I doubt very much that I would see one of these to work on it. I have seen them in person many times but without a wrench in hand.

IMO all Mercruiser engines coming out soon will have a 170 degree or even a 175 or 180 degree T-stat installed from the factory. Also possible other OEM marine engines will as well. FYI