C Mahillon & Co.
De Prins Gebr - Gebreveteerd, België
De Prins Gebr - Gebreveteerd, België
I do see some chatter mentioning "De Prins Freres" or "De Prins Bros." Just including this as a search term.

An alternate spelling is "Gebrevetteerde." Google Translate says it means "certified."

De Prins was in Borgerhout, Antwerp, Belgium.

Really nice info at http://www.rjmartz.com/horns/DePrins_092
E Albert - Bruxelles
F. Van Cauwelaert - Bruxelles
P. Senecaut - Bruxelles
Ph. Valinthout - Bruxelles