True per blueprint built RPK's have the smooth cover. The Romanians only used ribbed covers as a stop gap measure when they built their RPK's and could in a sense be deem "correct from the factory" but not "correct to blueprint".
The RPK-74's are going to have a smooth top cover. I think they were made late enough in the game were that's going to be a given.
But the RPK's could probably be found with a ribbed cover, especially in combat (swapping out, etc.). I agree with Tomovich - I can also see both being used quite easily on Russian RPK's. The Romanian RPK's seemed to be found with both styles as a standard.