Big-t-2538 once bubbled...
This is one area where NAUI isn't all that great. I got no Cert. # until I made my DM rating. All that's on the card is cert. date, Instructor #, and location of cert. I think it would be a good idea to issue numbers, but who knows...could be a bit of a hassle too.
I don't think the numbers really mean that much, who actually checks them? PADI cards change everytime until you get to DM, then your number is permanent. IANTD gives you a number that stays the same from card one, and as you say, NAUI does something different.