M in circle - origin?

[IversenJ]

Dec. 1916 - Cover from Casablanca probably via Odessa - Moscow - Petrograd - to Helsinki, Finland,
Three marks on front: "11/6" in triangle on point and "62" in Square, both are well known censor marks from Petrograd. But can someone help me with the origin of the "M" in circle?

[IversenJ]

Ref: Michèle Chauvet og Jean-Francois Brun, Introduction à l’histoire postale de 1848 à 1878, page 170.
This was a control mark used by the French Rural mailman to mark the mail taken out of a mailbox.

[GibbonsB]

Thank you for this information.

[LuzA]

Good evening

Ref: Michèle Chauvet og Jean-Francois Brun, Introduction à l’histoire postale de 1848 à 1878, page 170.
This was a control mark used by the French Rural mailman to mark the mail taken out of a mailbox.

It is not easy to consider the transposition of the "origine rurale" of the 1848 à 1878 Chauvet book to this 1916 letter in Casablanca/Maroc.

I would say that the circle with the M mark is most likely what is referred in the Bourguignat (2010) book (1) about the mail control of the Maroc workers and militar indigenous.
Circles with the letters A, F and M are known according to the referred author.

(1) - BOURGUIGNAT, Jérôme (2010). Le Contrôle Postal et Télégraphique Français Pendant la Première Guerre Mondiale (1914-1921). Académie de Philatelie.