French Censorship - strange triangular mark "ON"?

Hello Everyone, I'm quite new to censorship letters, so please forgive if there is a very obvious answer. got the below cover with seemingly french military censorship.
questions:
- can anyone say something about the round postmark ("... Postes") ? is this a military postmark? is the
- can anyone say something about the triangular postmark "ON"?
many thanks!
Peter
questions:
- can anyone say something about the round postmark ("... Postes") ? is this a military postmark? is the
- can anyone say something about the triangular postmark "ON"?
many thanks!
Peter