On my 09C (Norwood) car, two were stamped and two were not. All were mounted with dome up. However, whoever applied the sealant did a pretty sloppy job, as it was strung out over the floor pan. Must have been in a hurry.
Also, I was in Legends at Nationals last year, and did not lose any points for the two that did not have stamps. The judges can tell, however, if the plugs are original or repros based on the texture.