As mentioned, AIC only stamped some of their
cels. Cels from order shows aren't. And cels
from some of their latest anime aren't either.
In fact, only cels that were packaged by AIC to
be sold were stamped. (That's why Blue Gender
cels aren't : they already moved to CGI and didn't 
want to spend time packaging cels. They just bulk 
sold them "as is" : Whole unsorted sequences with 
all layers/drawing/background,...)
As for the other shows, there is both stamped and 
un-stamped cels depending on how they were 
released on the market.

Naturaly, that won't prevent someone from re-using 
a stamped plastic sheet to make a copy of a cel...

Only cels from the anniversary event of Studio 
Pierrot have stickers on them. So that's very 
little... (and naturaly, there is all the 
reproduction that they made for the anniversary)

Many Vampire Hunter D cels were sold before they
started stamping them. In fact, the studio sold a 
bunch directly at the very begining. All the VHD
cels in Japan weren't stamped. Some sellers said 
that they would stamp the cels they still had but 
I dunno if it ever happened.

Cheers,
Yann Stettler