Actually, the regulations in the U.S. have changed. They used to allow up to a half karat under the stamped value for a cast or solid piece. There was an additional half karat allowance if the piece contained solder. The cast or solid portion still had to be within a half karat of the stamped value, but the total allowance for the entire item was up to one karat below stamped value.
The regulations were amended in 1976 to allow only 3 one-thousandths deviation on the solid portion and 7 one-thousandths on the entire piece. These changes became effective in 1981.
There are two problems. When they first became effective, some manufacturers, proud that they were complying with the new regulations, started stamping their wares with a "P", such as 14KP. But there was no requirement to do this, so some just continued to use their old 14K stamps. Now, over 35 years later, if you're buying old jewelry, there is no way to know if the piece was made before or after the changes.
The other problem, as snoman pointed out, is that anyone can buy a 14K, or 14KP, or any other stamp, turn out inferior pieces, and stamp them themselves. Of course there are penalties if someone is caught, but there is no effective enforcement on the little guys.
As Nick said, the only safe bet is to learn to test.
Dave
