As to the variant choice for our V mark - way back the spits line was rearranged to give a better representation of spits at any given time frame of the war. Because many spit model served in parallel, our old super-spit V was a later model that our spit IX, and we were missing spits between the mark I and mark IX.
It was then that the models VIII and XVI were added and covered the late Merlin models, spit V was derated to a 1941 model and our spit IX represent the 1942-1943 "stop gap" mark IXs before the introduction of the mark VIII. As said by Karnak above, our spit XVI represents the late IX models.