My guess is that a French bike imported to the US in that time frame would probably have used 27" wheels unless it was very high end, in which case it came with 700c tubulars. So, 27" is my guess.

However, depending on brake reach, even if built for 27" wheels, 700c rims may work, since they only need to reach 4mm further than they would to accomodate 27" rims... Rear spacing is likely to be 120mm (5 speed) unless it has already been spread by someone else.