As for Apollo class vessels, the T'Pau has no canon class identity and in any case does not have a NCC registry. The Ajax has no class identity, making her low registry irrelevant to the argument. The same goes for the Clement, whose registry appears to be noncanonical as well.
In the episode “Brothers”, there is a Starship Mission Status chart. The chart identified the class of the Ajax
-class. The registry for the Clement
was seen on a causality report seen in the episode “In the Pale Moonlight”.