That is the repro left side brake bracket. It is exactly the same as the original. It is one of the brackets Frankster made. I sent him two of my original ones so he could repro them and in return I was to get my originals and a few new ones. Unfortunately I never got my originals back since he lost them. :doah:
Both the brake brackets and the swing arms are hard to find. That's why I made those comments on the thread of the frame you bought.
PM me if you are interested.