Prima,

I found this about the Canadian Kennel Club;

Many breeders tattoo their litters. These numbers are registered with the Canadian Kennel Club. This number consists of the breeders kennel number, the litter number and the puppies number in order of birth. These numbers can fade as the puppy grows and the skin stretches. Your local veterinarian can arrange to do a tattoo for you and your pet. The usual sites for these tattoos are on the lip, ear, and stomach.

Maybe it's for the breeder's info & tracing.