The CIP proof is basically needed that the police can tell it's a 7.62x53R rifle. You see, in Finland there is practice that when non-collector buys a gun police needs to inspect it so that it really is like you told them you were going to buy. However, with their normal expertise they simply cannot tell the caliber if it's not stamped to the barrel. That should say something.
Not all Finnish Mosin-Nagants are CIP approved though. Only those sold for non-collectors by the gov't after mid-1980s-something were cipped. Collectibles aren't cipped as that ruins their value.