It is CP as in Company Proof.London proof house is operatd by members of the worhipful company of gunmakers.
I disagree, and agree with Miller that the letters in the proof mark in question are indeed GP, based upon the information in Wirnsberger's book The Standard Directory of Proof Marks
. On page 99, this is written:
"On 14 March 1637, the gunsmiths banded together and founded the Worshipful Company of Gunmakers of the City of London. this name has been retained to this day, .................
It was at this time that the Crown over GP was first used and to this day, this is the mark of the London proof house..............
In the year 1670 the charter was confirmed and the power of the organization was extended. The Crown over GP was retained and yet another proof was added - the inspection after proof - and this was marked with Crown over V."
That is a direct quote from the book. There is no mention in the book of any English proof mark containing a CP, that I can find.