I think it's a postal cancellation, and the "PB" is part of "NPB", which stands for Newspaper Branch. (˝d was the postal rate for newspapers.) Alcock and Holland provide this illustration of one of a large number of different NPB cancellations. It's not very similar, but it shows the typical large P and small N and B:

EDIT: R C Alcock and F C Holland: British postmarks, a short history and guide, revised edition, 1977. Indispensable!