I just transform @None’s (Laure) comment into an answer in order to remove this question from the queue of unanswered ones (as @Quidam suggests). [A couple of things were added.]

A (or À) (fill in with the name of the town where you were born) N° Départ. [1] stands for Département (where you were born).

The final dot makes clear it is not the word départ but an abbreviation of a word starting with départ-. (thanks @jlliagre)

‘Département‘ is an administrative subdivision, and they want the number, not the name. It might not have an equivalent in your country. Check this article on Wikipedia. You have found out correctly for the rest.

[1] An alternate abbreviation for département can be dép.. [Thanks inboxus2ébastienC for pointing it out.]

Another one is dépt. which is less ambiguous. [Thanks @jlliagre for pointing it out.]

[Cf. http://www.guide-typographie.com/abreviations.htm%5D

Please note that the correct abbreviation for département is dép.
Départ means departure in French.