Since the question's been raised again recently...
 
The real story on those two blue dots in your ferret's ear
                   by Dr. Judi Bell
reprinted by permission from the WNYFLFA Ferret Chatter, Sep/Oct '95
 
Marshall Farm spays, neuters, and descents ferrets at 6-7 weeks of age.
Because they heal so quickly, and regrow the hair at the incision sites
before they leave the farm a week later, a system was devised to prevent
unaltered kits from accidentally being sold with the ones that had had
surgery.
 
A simple tattoo, consisting of two black dots, is placed in the right ear at
the time of surgery.  The dots mean that the ferret has been both descented
and either spayed or neutered, depending on sex.  As the kits grow, the dots
grow and soon look blue instead of black.  The tattoo was not originally
intended to identify Marshall Ferrets for anyone but Marshall Farms
personnel.  However, these dots have become an unofficial trademark of
Marshall Farms' ferrets.
 
There is nothing distinctive about two simple dots, which can and have been
copied by other ferret breeders.  Most of the time, if you see a ferret with
two black dots in its right ear, it will be a Marshall Ferret.  If the dots
are any color but black/blue, or are in the left ear, of if there is a
single dot in either ear, the ferret came from somewhere else, even if the
owner has documentation showing the Marshall Farms name.  Documents can be
copied or transferred, and the ferrets have no unique marks to absolutely
identify them.
[Posted in FML issue 1477]