Good evening Martin,
I've had a look at the two major sources regarding the 55th Territorial, i.e. Souvenirs d'Alsace, by Major Fleutiaux, and Dix-Sept Mois de Campagne, by Doctor Brizard.
Fleutiaux mentions no stay abroad, and according to him, the 3rd battalion of his regiment was disactivated on January 3rd, 1917, at Clefcy, in the Vosges. Moreover, after checking the list of the regiment's KIA, no mention of a man killed abroad either.
As for Brizard, he only details the disactivation of the 3rd battalion -which was his own, by the way- indicating that it started on January 4th and was completed by February 9th, 1917. But there again, nothing is to be found in his testimony about a stay abroad concerning any fraction of the 55th Territorial.
Best regards,
Eric Mansuy
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"Un pauvre diable a toujours eu pitié de son semblable, et rien ne ressemble plus à un soldat allemand dans sa tranchée que le soldat français dans la sienne. Ce sont deux pauvres bougres, voilà tout." Capitaine Paul Rimbault.