I finally tied together all the loose ends of the Fidanza/Fidance family of Marseille.Even though this isn't your family, I thought you'd like to know how I found the full birth records for Marcelle and her siblings.
It turns out that most of the RECORDED dates were 1 or 2 days after the actual birth.The records may say "today" (audourd-hui), yesterday (hier), or day-before-yesterday (avant-hier).
To make things worse, Marcelle's birth month was incorrectly entered in the index as JUNE.The actual record says recorded 11 JANUARY 1898, born 2 days earlier.
So Marcelle was born 9 JANUARY 1898, which agrees with the Social Security Death record of Marcelle Crepeau.
Hopefully this new information about the lag between the actual birth and recorded "Acte de naissance" will be useful to others.I wouldn't have found this Marseille resource without your help.I'm very grateful to you.