Shell width is only very loosely correlated with thread spec. While 68mm is the most frequent width for French-thread shells, anything is possible.

You say the original bottom bracket was TA. TA and Stronglight differed from most other bottom brackets in using the same axle for both 68 and 70mm wide shells, and varying the thickness of the cups to make up the difference. If your Bertin is set up to use a double crank, that would mean a#344 spindle.

TA cups use rings to indicate thread spec:



And TA lockrings use knurling to indicate thread spec: