I'm new to this too. Came across this site in my search for info on my husband's sax.
His sax is marked Antoine Courtois. Courtois lived around the same time as Millereau and was known for making brass instruments but not saxophones. The markings on my horn are:
Facteur Du Conservatoire
8 Rue De Nancy, Paris.
This information is engraved inside the bell of the horn.
Brevete' means certified or patented so you may try to check French patents. Though he manufactured saxophones, Millereau may not have necessarily made your horn. The H. Schoenars succ on your horn may mean they were certified to use the Millereau name or patent. If the Millereau Co. or it's successor did make yours, maybe they made mine too. I believe Facteur, means postman, so da Conservatoire, meaning the Conservatory, would be part of the address. I am assuming of course that the Fr. abreviation stands for Facteur as spelled out on my horn.
In the case of my husband's sax, I've discovered Courtois is the oldest of all current operating musical instument manufacturers and is part of the JA Musik Group. The Courtois saxophones are being made today by B&S, another company in the JA Musik Group and they are made in Germany. So perhaps your horn has a similar history and is still being made by a current company.
My husband bought his sax at a music store in San Francisco as a high school student in the mid 1950s. I know he already had it when I met him in 1958. The markings on your horn seem very similar so that may help date it, at least give you a starting point. I found the more current information on his sax at a music store site so perhaps you might try that.
Good luck on your search.