
The NDTR was originally founded with the help of the GSD League with The Late Rory Frost as themain driving force
There are different numbers that are put between the tattooists's ID letters that indiicate that the dog was tattooed as a puppy 1 rotties, 2 dobe, 3 working breeds(including the pastorals)etc it means that there are more numbers available for use.
So yes GSDs were the 1st to be done then as more people wanted different breeds done the number codes were included.
Originally it was just to indicate if the dog was done as an adult or puppy