They didn't use it from the beginning. You can't 
judge the authenticity of an AIC cel by only the 
presence or lack of the stamp. I collect a lot of 
AIC cels. . .

For example: I've never seen a stamped Vampire 
Miyu OVA cel. But a large number of Vampire Miyu 
TV cels were released before they started using 
the stamp, some were released after. The result? 
Only about 1/3 to 1/2 my Miyu TV cels have the 
AIC stamp on them. My count for Nightwalker is a 
little higher -- over 3/4 of my cels are stamped. 
But still, some of the cels for the show were 
released before the stamping began.

All the cels I've seen from Now and Then, Here 
and There are stamped, but I've never seen an El 
Hazard OVA cel with the stamp. On the other hand, 
El Hazard Alternative World cels (which were 
relased much more recently) *almost* always have 
the stamp. *Almost*.

For Ah My Goddess, I don't think any of my OVA 
cels have stamps on them. My movie cels are 
almost all stamped, but even then I have two 
which are "low quality" images and are not 
stamped. Those cels by far aren't valuable enough 
to have been worth a fancel artist's time to 
forge them (I obtained them for about 500yen 
each) and, for that alone, I doubt they're fakes. 
They're also by far not valuable enough that 
anyone would bother to steal them from the studio.
So why no stamp? Who knows. . .

There's nowhere to look to find out when and what 
cels AIC started stamping. If an AIC cel doesn't 
have a stamp, that might only mean it left the 
studio before the stamping started. . . Or maybe 
it was deemed not worthy of a stamp?

Another point about the AIC stamp is that 
sometimes the stamp is on the backgrounds too, 
albeit, it's rare to see. Why stamp only some and 
not others? I get the feeling that AIC doesn't  
completely religiously use the stamp.

Many Sharp Smiles,
--Drac