It's an America waet Express/US Airways Express flight operatedby Mesa Airlines. Its not a codeshare as a major airline employee would be thinking- like US codesharing with United, or Lufthansa, or Big Sky or whoever. In fact it is not a codeshare at all, it is a capacity purchase- a US/HP flight number that happens to be operated by another carrier- most Express agreements (except the owned ones) are really capacity purchases/wet leases. However, a recent trend in the regional airline industry is to refer to these agreements as "codeshares", as if these "airlines" would have been operating these flights anyway and they allowed the major to "codeshare" with them. For example, I overheard a pilot for one of the affiliates referring to US as thier "codeshare partner"... maybe it makes them feel important, but codeshare is technically the incorrect term.
Now none of that changes the sad fact that the customer thinks they are on US Airways when they are on these flights. But have no fear! The company is making great strides in strandardizing the service experience between the 20 Express carriers and real US Airways... they are bringing US Airways down to Mesa levels.