Barks lithographs question. What does PP mean?

[Daniel van Eijmeren]

In a private email, someone asked me: 

"I received a Carl Barks litho which is a gold plate edition from Another 
Rainbow, it is signed in pencil with PP then the number of the series. 
What does PP mean?"

Does someone know the answer to this question?

(Maybe "PP" means something like "plate production"?)


Best wishes,

--- Daniel

"I knew it! I knew it! They'll ROB me! I'll be a POOR old duck!"
(Which one? Hint: part of the story's title is mentioned in the quote.)