Yep, that plate is called the "Plaque des mines". Mopeds also have them, so I'm guessing all 50cc vehicules had them sisnce they werent registered?
IT is the date of "type approval" as you say, and then you get the "feuille des mines" which has all of the original specs on it. It's good, because its the only paperwork mopeds have, and the french law says "you need indicators if the vehicule originally had them" and if you don't have any, you need to prove it, and these papers usually state it.
JP