that serial number could well mean it's an American made early (80 ish) MX-II frame. later MX-II's and series 3 frames were built in Japan and had the redline stamp. i had a MX-II that had a redline stamp and serial starting with RB, telling me it was 1981 built in Japan.
ask the fella if it has holes from the down tube, seat tube and chainstays in the bottom bracket shell, basically 4 holes. if it does it will be a series 3 if it only has 2 it will be MX-II. dating the early American made redlines is difficult as there is no clue in the serial number. obviously the higher the consecutive number the later it will be. the Japanese made frames were a little easier with the R? serials.
that's a nice frameset, good luck with the auction
