A terrific find, a beautiful job of bringing the pipe back to life, and excellent detective work.
I've dim memories of visiting their shop (and that of Balkan Sobranie too) as a teenager at a minor English boarding school. And I must say it is hard to believe how civilized the world of London tobacconists once was.
I too happen to be the proud owner of a Sullivan and Powell pipe (a large, almost unsmoked Canadian), that I thought an irresistible bargain ($53 including post), pretty much ignored on Ebay about ten years ago.
I'm inclined to think mine may be a bit earlier than yours. It has somewhat different nomenclature (which includes the shop's full name and address, the legend "London Made," and "815"--which I presume to be a model number). And my provisional guess has been GBD--a maker I know little about (notwithstanding Greg Pease's eloquent praise in "Pieces from my Collection").
In any event, thanks so much for posting this--it's renewed my curiosity--even though I may now be wishing I'd also socked away a few tins of Gentleman's Mixture to keep the pipe company.