The "two blue in right ear tattoo" holds up for altered ferrets--usually.
However, I have at home two ferrets that were used for demonstration
purposes by a neonatal physician (to demonstrate the size of the windpipe of
neonatal children to nurses and doctors in training, they aren't hurt a bit)
that both have two blue tattooes in their right ear.  Both were purchased
from Marshall Farms.  And both were UNaltered as well as not descented.  I
had both neutered but NOT descented.  It appears quite common to receive
unaltered ferrets, especially to lab-type situations where the ferrets are
older, that have the tattoes.
 
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Amy M.E. Cada
[log in to unmask]
Grad student, Behavioral Neuroscience, University of Texas at Austin
Asta--2 yo male scottie-agility, tennis ball, bath, and soon-to-be flyball
      nut
Wylie--2 yo rescue male wire fox--practicing obedience so we can do agility!
Hobbes, Zowie, Foster, and Farris--the "long kitty" ferrets
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[Posted in FML issue 1478]