The Sint B 12 is meant to be a pretty good substitute for Westy 12 (it's just not listed as an official Trappist ale). They share a history; without looking, I think that Westvleteren used a larger brewery down the road at Poperinge pre-WWII, before they moved operations back to within the confines of the monastery. That brewery (the one down the road) became the St Bernardus one and pretty much stuck to the Westy recipe. Or something like that.