Yeh, "B" represents an acoustic bass guitar in Martin's system. So a B12-35 would be a 12-string bass guitar with a D-35 style appointments. A not impossible, but pretty darned unlikely beast. Probably supposed to be a D12-35.

I played a D12-35 that belonged to Scott Boyer while it was in a friend's music shop waiting to be picked up after having been repaired. It sounded great, but it was in the shop, which seems to be where a lot of Martin 12's made before they started using adjustable truss rods seem to spend a lot of their time. If you want a 12-string from that era, look for a Guild.