A quick search for comic postcards and you come across a whole variety of numbers: R5432, A5432, No. 666, G7... and so on.
Do people actually search for postcards by keying in these numbers? Probably not. If it was a group of ten and someone was missing one there would be a chance that someone would search for it this way.
Most people search by what is actually ON the postcard - after all the postcards are divided into comic, topographical, and so on, and then into various artists. The customer is buying a card, not a serial number, likewise a book buyer buys the book, not the ISBN.
Given that there is a limit to the number of keywords you can place on a title, why waste space by sticking numbers up.
1 to think about...