I am less familiar with French material than Vietnamese and I have a couple of questions that I am hoping readers can answer.
1) Is the notation in the upper left corner a shorthand return address (M919/4B)?
Philippe Drillien writes,
I do not think it's a return address. According to the French regulation, every French administration must own two books . In the first one, you must write, every day, all the mail you receive; each mail is given a number of arrival; often, the number is preceded or followed by one or more letters. Usually, the letter concerns the service or the name of the addressee. The second book is for the mail you send and works according to the same way. M919/4B could mean:2) The last word in the top portion of the unit cachet is unclear (after Tirailleurs). Perhaps Algeriens?
It is the 919th mail registered by the secretary M. It has been written by 4B