It's a number that identifies when the die for the metal insert was made. It's usually within a few months of when the chip design was introduced, but need not be.

Some chip designs are found more than one date and are identified in The Chip Rack with different catalog numbers. Look at the Stardust chips from the 1970's, for instance.

Some chip designs are found with the date on one side only. I don't know of any particular reason for this... just the way the factory decided to do it.

Another variation found on some Paulson metal inlays is the color of the anodize. The metal used for the inlay is aluminum, and if a die is added to the solution during the anodizing process, the result will be a different color surface finish. Look at the El Cortez chips and you will find silver and gold variations of one of their chips, I believe it's the $25. Other colors of anodizing used on some casino's chips include blue, green, and black.