AFAIK, the "B" model originally meant top surface only dive brakes, foam core wing, large tailplane, and provision for optional water ballast, including higher gross weight and (I'm reaching on this one) higher VNE. I don't know that many came over with ballast tanks so for most of us, that simply meant the drawing that showed where it was OK to cut holes in the main bulkhead and landing gear box for the dump tubes; we fitted U.S. bags and homemade valves and tubing.

Having said that, I believe that all it took to upgrade a 201 to a 201B was some paperwork, so seeing "B" on the documentation might not be definitive..

Chip Bearden