Patience, patience! I just got home with the bike. Enjoyed meeting the very nice young couple who were selling the bike. Apparently it had belonged to her Grandmother (which made me feel instantly ancient since I bought a similar Stumpie in 1983 as well)!
It is a Stumpjumper after all. The serial number falls right in line with Stumpjumpers produced at the same time, and it shares all of the other distinctive features with Stumpjumpers produced in the same era. The only difference is the Mixte frame. I know I've never seen or heard of one before this. I can't imagine that there were too many of these built? Anyone else ever come across one in the flesh??