Raimey: it's usually Acier Siemens Martele and has been found on Francotte and others.

About the middle here is a Masquelier with barrels by Falla
http://www.littlegun.be/arme%20belge/artisans%20identifies%20ma/a%20masquelier%20gb.htm

Falla listed Siemens-Martin via Joseph Cap



I'll bet Cockerill Sambre was providing the new fangled steel