It doesn't depend of the serie but on when they 
sold the cels. So there is OVA cels that have 
their stamps and there is TV cels that don't have 
it. (also some cels were sold without the stamps 
even after they started doing it : for example, I 
got many Miyu TV cels from the studio and they 
don't have it)

Cheers,
Yann Stettler