I have cels from the 1st OVA series, 1st TV 
series, and 2nd movie stamped.

For the OVA and 1st TV series, the op cels are 
usually stamped.  My Mihoshi Special OP cel (2nd 
special from 1st OVA series) and Sasami OP TV cel 
are both stamped.

I also have a Washu cel from the 2nd movie that 
is stamped.  My other 2nd movie cels are not 
stamped, however their backgrounds are stamped.