Mysterious signs, confusing advert

I’ve walked past this premise in Bangor many times over the past twenty years or so, and always wondered about it. The combination of ‘self-service marketing’—which seems an odd thing in itself—and ‘hosiery factors’—whatever they are—fries my brain.
Some dedicated sleuthing hasn’t revealed an awful lot, though in an advert in the Mid-Ulster Mail from 1 March 1990, they declare themselves to be:

But this just provides more bafflement: surely the suppliers of the finest hosiery in all the land would be able to spell the name of the thing they sell?