If anyone needs a French L. Maxant of Paris altimeter, there is one for auction on Ebay in France at:
French L. Maxant altimeter It is in rough condition, but it looks like the outside at least could be restored without too much trouble. The seller does not say anything about the condition of the inside.
Usual diclaimer: I have no connection with this seller and I do not know how original this altimeter may be. I am just mentioning it here since I know there are a number of guys working on restorations and reproductions here and great war era instruments are not easy to find.