Or Proved and Viewed. There are two stamps. The "P" meant the barrel was proved, that is, that it had been fired with a heavier than standard charge (a "proof" load) to make sure it was made properly and wouldn't blow up in service. The "V" meant that after it was proved it had been "viewed" (inspected) to see if any cracks or other flaws had been brought out by the proof. Two separate stamps were used because in mass production, guns might be proved at one time and not viewed until some time later.
While the use of modern steel makes proof testing almost a formality, it is good to know that it is still done and that the firearm you buy or are issued has been tested and passed.
(The VP mark, standing for Verified Proof, was used by Colt as its single proof mark.)