In 2014 or 15, after a year or so in the FAA, I had 2 months added to my seniority for contract tower time from 2001. This was not easy to prove, but for some reason I still had my W2 from Serco. I don't think they became unionized until several years after I left. I jumped 2 or 3 people, but they didn't seem to mind. It felt dirty, but it was legit. I went back to the DOD several years later where SCD rules the seniority list.