Lee Kennett wrote in "A History of Proof Marks, Gun Proof in England" (Gun Digest 1977 Edition, p.161) that "Another variant, and a more baffling one, is the frequent appearance of a crowned P in conjunction with the usual view mark. The theories concerning this mark are many: that it is the result of a defective London die, a mark of a proof house in Liverpool, or in Dublin (where it is known that arms were proved in the 18th century), or possibly the mark of private proof in Birmingham. That it is the mark of proof elsewhere seems plausible, for the London authorities from time to time seized arms bearing other marks."
Markus
