Agreed. Which one is anybody's guess. Maybe some folks on that site could help you out. Saw more than one seat mount in those pictures so you may be right about the year thing. Good luck and get busy restoring it. Spring's coming!i think its a model m-something. i found a site that shows many that look like mine that all start with m so i think i could be possible of that serries.
http://www.speedextractorinformation.com/MSeries.html