The "BW" has been the subject of much speculation over the years, but Bode Willumsen was a new theory to me. The information I have, which is from a renowned Swedish glass expert, is that the combination B-W refers to Elis Bergh models with engraving designed by Gunnar Widholm.
Having said this, I personally do not believe that it says "BW" or that it is Kosta. To me it is more likely that it says "pu", and that it is a Palmqvist piece from Orrefors.