I'll be honest- almost anything I know about Bertins comes from Jim's site- and with the proviso that I don't know whether the features that suggest to me that this is a Bertin are unique to Bertin, or were widespread among French builders.. here is the evidence:
1)Bertins are documented as having been routinely built with Reynolds frame tubes matched with Vitus fork blades. OP said the forks were Vitus, and showed us a frame tube stamped 'butted' in an earlier thread, which is almost certainly Reynolds*. I don't think that English word would get stamped into a French or Italian tube, although I'm happy to be corrected on that point.
2)Bertin seem to have been big on Prugnat 62s for some time.
3)Bertin seem to have foregone the practice of stamping serial numbers on their frames for some time.
4) There is a photo of a Bertin C34 'cross in the gallery at Jim's site which has just this same internal front brake cable routing. Unfortunately it's not a very sharp photo and the frame is black- severely compromising recognition of other features- lugs etc.
*I can show the "REYNOLDS BUTTED 16/13" stamped into my steering column if anyone is interested.