Hi David.
I don't think that it had anything to do with Jet tools. Wadkin made a bunch of machines with just 3 letter names. the Wadkin PAR was the combo two sided jointer and two side planer. Maybe short for Planed All Round. The Wadkin SET was for the "Single End tenoner" The name Jet was probably "Jointing end tenoner" or some such thing.
I really like all three machines and would be quite happy to have all three.
But for know i have theses JET machines and i am quite excited to get one together as i have a couple of projects in mind for them.