Back Street Pins
A magazine, published by Toplis & Dibbler of Monkey Street, which is strictly for the adult acuphile. An idea of its content might be gleaned from knowing that Toplis and Dibbler's other listed publication is Girls, Giggles and Garters, and also that Dave Stamper refuses to have it in his shop, as his is a reputable business and does a lot of selling to kiddies. It is also referred to, in Going Postal, as Back Alley Pins.
Anything on print sale marketed with the prefix Back Alley... or Back Street... is seeking to present an image of being edgy, dangerous, mean to mess with, and offering a frisson of danger. Several magazines have traded on this, dealing with the art of tattoos and bodily modification, or motorbikes, or both at once. They are usually distinguished by having lots of pictures of scantily dressed girls lounging around on motorbikes - not theirs, of course, the mere idea that women have a right to ride them (except as pillion passengers)! Unthinkable. Or else there are lots of pictures of girls showing off their tattoos and/or piercings. Sometimes even lounging on motorbikes while doing so.
The ur-example is long-standing motorbike/scantily clad women magazine Back Street Heroes
So a guess can be hazarded as to why Dave doesn't want it in the Pin Emporium...