I have read about him and seen some mentions of basses. I think he was maybe a dealer. It is also possible that basses with his label were made by others. Paul Bisch basses and bows for example were made by others in that period as well. I have and have seen one other Mangenot bass from c.1900 that were similar. 3/4, round back, dark varnish, sloped shoulders. Typical solo model of the day.
Look similar?
http://www.kensmithbasses.com/doublebasses/mangenot/