In common with my (full Campy NR) Stella are chromed lugs, wrap'round stays, and "swallow-tail" end treatments. Differences include: Mystery has TT cable guides, different (style?) lugs, chromed socks (my Stell has only DOs), smaller seatpost, and according to pic, very long chainstays. Also, all 70's Stellas I've run across had more elaborate/better than avg paint but not Mystery. Stella good guess but seeing side-by-side I'd say some other french/swiss (sport-tourer). Hope seller posts BB thread to at least confirm french.
Jack Romans
Sacramento, Calif.