It is CP as in Company Proof.London proof house is operatd by members of the worhipful company of gunmakers.

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

Further examples of early English provincial marks are recorded in Neal & Back’s book “Great British Gunmakers 1540-1740”. They include a pair of pistols stamped with oval marks, crowned P & V and a makers’ mark, by Cornforth c1715, p.389 and a pair with similar marks by D. Newton c1715, pp.380-2. Several firearms by Thomas Richards dated between 1749 & 50 are shown in the companion volume “Great British Gunmakers 1740-1790”, pp.119-120, bearing the crowned P & V and his own T.R mark.
As the 18th century progressed, private proof marks became more common. The usual marks are a crowned P and V contained within an oval (occasionally without the crown). Their similarityto the proof marks of the London Gunmakers Company suggests that this was an attempt to demonstrate that their barrels were properly constructed and proof tested and thus just as good as the London product. The London gunmakers objected most strongly to this practice and confiscated any examples found in the city and its environs as well as fining the people concerned. In 1717, the Gunmakers Company offered a 5 guinea reward in the London Gazette for information concerning the use of “false or counterfeit stamps in imitation of ye sd company’s mark” (“Dictionary of London Gunmakers 1350-1850”, H.L.Blackmore 1985, p.22)

The crowned P & V proof marks are generally referred to by collectors and dealers today as “Birmingham private proofs”; this is almost certainly due to the fact that many known Birmingham pieces are found with these marks. Some provincial makers used very similar marks, suggesting that they might either have sent their own barrels to Birmingham for proof or else bought Birmingham made barrels. This became more common after about 1760 and certainly by 1800 all manner of firearms could be ordered from a Birmingham gunmaker, complete with whatever signature was required

Interestingly, the barrels of several guns signed by well-known London makers are stamped with the crowned P & V mark described above; for example on a fine sporting gun by John Harman c1730 (see “John Harman c1693- c1760”, 18th Park Lane Arms Fair catalogue 2001); also on a silver mounted cannon barrelled pistol by Joseph Clarkson Jnr., c1765 (private collection) and the pistol illustrated in Figure 16 signed Turvey, London, c1775. No explanation for this anomaly has been found.