The first image on the left (stamp A) shows a stamp with a larger printed area and a small frame and the one on the right (stamp B) which I believe is the norm has a smaller printed area and a larger frame.
I have not actually seen these stamps my self, but I do remember Douglas Myall writing about them in his Deegam Report some time ago. These pics were borrowed of the web.
THE MORAL OF MY STORY IS :
When checking your kilo ware, before discarding any as duplicates it will pay to double check these stamps as Stamp A is now sought after in fine used condion and is selling for as much as £100 for a nice example.