Well, still can't be 100% sure and I would have to strip the paint to figure out what tubing it uses, There would be clear stamps underneath it. It is lighter than my GS with Vitus 888 tubing, with the same components (and exactly the same size). So it would have to be V172, R531 (as the seller claimed) or C SL (which it's not).
But I had a look at every 78/79 Motobecane photo and catalogue I could find (not a bad task), comparing various models, lugs, forks, seat stays, BB shells etc. Definitely Bocama lugs and they are cut out, which would leave us with Grand Jubile, Grand Record, Le Champion, Team Champion - or the continental Europe equivalent (CT/CTM 3, C3, C4,C5). GJ and GR had short ones, so LC and TC. Then looking at the seat stay caps, definitely not TC / C5. There are some models in 1978 which seem to have similar seat stay caps (C2 and C3 - Grand Sprint / Grand Record, at least as much as I can see in the catalogue, that's not always consistent with photos of C2 and C3 I found elsewhere), but they also have short Bocama lugs. This combination leads me to believe that this blue frame might be C41 (or Le Champion equivalent) from 1979.
Comparing the way it rides to my previous Grand Sprint frame, I have to say there doesn't seem to be much of a difference. GS seemed to be a bit happier taking narrower turns, but then I was riding it everyday for a year, whilst I'm still getting used to the current one. It is different, but can't put my finger on how exactly quite yet.