DSO is the district that originally ordered the car. Each district has it's own 2 digit number. 72 is San Jose, 71 is L.A., 74 is Seattle, 51 is Denver (for our years anyway), etc., etc. I'm having a brain fade because it's midnight here, but I think DSO is an acronym for District Sales Order. Someone jump in and correct me if I got it wrong.
Steve