The prison number for each of the Beagle Boys is some combination of the numbers 1, 6, and 7, a holdover from the Carl Barks Uncle Scrooge comic books, wherein they were identified only by their numbers.
In Carl Barks' stories, the Beagle Boys have no individual personalities and they all look the same. They are only mentioned by their numbers and they all begin with "176". The only exception is the founder of the Beagle Boys, Blackheart Beagle, unless you count the fact that 176-167 is very fond of prunes. Blackheart never appeared in Ducktales and was replaced with an original creation, Ma Beagle.
In the comics Donald Duck was not in the navy, and always accompanied Uncle Scrooge, Huey, Dewy, and Louie in the their adventures. However, the shows creators thought that he might steal the focus from Scrooge and that his voice would be to hard to understand.