US Airways gives the two letter designations JS for PSA and EN for Piedmont but those codes are NOT IATA codes - they are only used internally at US Airways to designate those two airlines. The two airlines don't officially have an IATA code and they fly using the US code.  
 US Airways gives a two-letter code for all of their express airline affiliates and most of them are the actual IATA code. One notable exception is Trans States Airlines which IATA Code is AX but US Airways labels them TJ for internal reports and identification.