Chris,
Nice looking frame. My first question would be are the top tube cable guides original to the frame or did CyclArt add them? I cannot say I have seen that type of guide or guides in that location on a Lejeune. And are the decals reproductions of originals? I have seen at least one Lejeune where CyclArt put later decals on as that is what they had to offer.
Assuming the decals are "correct", and who is to say what is correct for Lejeune when you see the plethora of markings they used, I am thinking it is even later than late 70s. More like 80s. That seems to be when the L heart appears, in my observations.
A very generous French member provided me with a catalog recently. (Thank you again Christian) With it are several sheets of detailed description of various models. One sheet with a June 1980 date mentions Model 102, Course Spidel Competition. An all 531 frame with Spidel components, presumably top of the line. No guarantee that is what you have, but that is what I can offer.
Having said that, this frame could well have been used with a Champion du Monde model.
Probably created more questions than answers with this.....
Lynn Travers
Hazelwood, MO USA