I joined the Delta Frequent Flyer Program in 1981 (or maybe 1982). My membership number is 201xxxxxxx. I remember that I got a 10,000 mile bonus for signing up. This was enough for a first class upgrade, which I promptly redeemed. I flew this past week on a mundate domestic route on DL. (It was the same route that I flew the first time I flew DL.) In the details of the miles from the flight activity, I got the basic flight miles, the COS bonus and the PM bonus. In addition to these points, there were 5,000 extra bonus miles associated with the flight, labled "SkyMiles Enrollment - 5000 bonus miles." It has been about 30 years since I signed up, so it seems a little late for an enrollment bonus. Any ideas what this is for?