I think you are probably right. I looked in Kosta 250 and thee were, to my surprise, I'll admit, some pieces pictured from the 1930's by Elis Berg that were signed "B###." Later on, it seems that Kosta was added to the markings on the bottom. The numbers appear to be sequential with B 228 dated 1931, B 376, 1932, and B 536, 1933, B 596, 1934, and B 1225, 1937, which would make your piece about 1934 or 5, I reckon.
Good find, although the motto if its not Holmegaard, it's not glass will have to be withdrawn.