Hi Dave
It does not look like Douglas to me. Douglas certainly had a similar mechanism especially on machines with dummy belt rim braking but Douglas were not the only manufacturer to use this design of brake and in fact your first choice, BSA, also had this type of brake in the 20s. The part number you have quoted also does not appear to match the usual Douglas format of part numbering. Apologies if someone recognises it as part of a Douglas model with which I am not familiar. Chris.