Chris,
After posting on Saturday I had a premonition I would not find a flasher box on my B12.....
...And the premonition was true, of course! With the later Italian regulations, and also the fitted separate (from stop lights) Carello flashers to go with them, there is no need for a flasher control box anymore!
But I presume you have now found the mounting points on your B21S for that marvelous contraption!