'B' prefixes cover three years of manufacture (1953,'54,'55), the particular year being determined buy the actual numbers...So for a more definitive answer on that you'll have to provide the numbers...If you don't want to do that here, though there's no reason not to, you can contact me off forum...
The 'S' suffix indicates 'sprung frame' which is a plunger model as you suspected...So, assuming the frame hasn't been restamped you have a plunger frame front with a rigid rear frame and other parts fitted to it....Plunger and rigid 'tele fork' frame fronts are dimensionally identical..Earlier girder fork frame fronts were different...
In terms of desirability the rigid frame versions is the more sought after generally...Ian
