a "mixte" or a simple "m" tag is not unusual for those records
In 1871, the treaty of Frankfurt ratified the loss of Alsace and Moselle to the new German Empire, which had been proclaimed a few months earlier in the Hall of Mirrors at the Palace of Versailles.
Alsace and Moselle remained annexed until the end of the Great War.
My assumption is that this tag is just saying they were not considered with the same nationality at the date this tag was added (probably in 1873 or later).
but it would be nice to have a confirmation from the Archives of Paris. Will send a request and let you know
CHOUKROUN ATTALI ATLANI