My RWS 48 has a B21 extendable cocking lever on it, Id bet the stock would work as parts interchange. Hopefully someone who has both can chime in. Been years since heard anyone even mention a B21.
i think what you might find is not all the clones are the same so that complicates things i have a BEC Eurolux 21, it is a side lever Diana clone, and the stock would never fit a 48-52 but is it what a b21 is all i know it is a big rifle