A carryover from the Pinnacle/Mesaba merger. They both used CR9 and CRJ before IIRC but while they were under different certificates they needed to differentiate which maintenance program and certificate a given a/c was under. So Pinnacle CRJs became CPJ and the Mesaba 900s became CM9 for Pinnacle and Mesba respectively (Mesba CRJs were CMJ). All the Mesaba CRJs were moved to the Pinnacle certificate and the Pinnacle CR9s were sent to ExpressJet (and are now coming back) but the designations were left that way.