With US, I seem to recall that the mainline aircraft (the Express aircraft have all sorts of different codes) bear the following two letter codes at the end of Nxxx string:
US (for US Airways)
UW (which was used on some of the airbus aircraft since NW has tied up some of the NxxxUS registrations)
AU (which is the periodic table symbol for Gold - no idea why they went that way, except if it had something to do with Alleghany)
VJ (stood for VistaJet - the marketing name for the ol' DC9-30s - since retired)
Other than when US and PI were integrating fleets do I remember any other codes being used (for PI, they seemed to have used a one letter code "P", and sometimes "N" - at least according to the fleet guide at JetPiedmont.com).
I don't think any of the codes used by US represents the owners of the aircraft (for example then, some would have something to do with RSA, or a bank, or whoever else owns the aircraft)