One thing I noticed is that a 82 model of Mistral had braze ons on the seat stays for a rack, whilst an earlier model didn't seem to have it. Of course it could have been made on order, but the catalogues of 82 clearly show the full bike having racks attached to these braze ons, whilst earlier catalogues showed the racks attached to the brake nut.
These details might help.
My 82 Mistral has this as serial number: U59218 (I read it from the forks where it was pretty clear, could be a 0 instead of U, but I am pretty sure it was a U)
https://www.flickr.com/photos/anidel/sets/72157643692675805/
Sorry to wake up this old thread
hopefully you've been able to date your frame.