I have actually never been asked to show my c-card when getting air fills. Even when I was new to the sport, no one ever asked to see my card to get an air fill. Most have asked to see it when I charter a dive boat though, just not air fills. But if they did ask you, I doubt they would accept a specialty card...technically there supposed to view your actual diver certification card and not just a specialty. Even though you need to be a certified diver to take a specialty class.