Masquelier made some nice doubles
https://www.littlegun.be/arme%20belge/artisans%20identifies%20ma/a%20masquelier%20gb.htm

Are the flats marked 70 (mm) for 2 3/4" chambers?

No, 'B' would not be the date code, which are shown here
https://www.shotgunworld.com/threads/belgian-shotgun-identification-guide.232252/

Did the gunsmith remove the stock and check the head of the stock for more cracks? In that it is already cracked, I would limit loads to 1 oz. if you decide not to repair the crack.