Can't help with any specifics, but my understanding is that the hammerless age saw a shift in actual gunmaking away from Scotland and other "provincial" locales to Birmingham, especially for boxlocks. So I'd guess the odds are better than even it's a Birmingham gun made for and sold by MacPherson's. I saw a fantastic boxlock just slathered with very fine scroll engraving bearing the name Gow's of Dundee, another venerable retailer of shooting and fishing gear that closed just a few years back.
It was almost certainly a Birmingham gun. From time to time I regret not buying it.
