Iran 10000 Rial (1980) Notes with "8" Stamp on the Back

Hello everyone,

I realized some of the 10000 Rial banknotes (P#131) have an “8” stamped at the back. It is always blue and at one of the margins on the reverse. It is mostly on the left, but I have also seen one example of it being at the top. Any idea why? I couldn't find anything about this on the Internet. 

Some examples:

https://katzauction.com/lot/324325

https://katzauction.com/lot/527941

https://www.greenappleauction.com/en/product/10187692/iran-10-000-rials-1981-aunc-p131

Thanks!

As far as I know, the  "8" Stamp is on a counterfeit note.

.

That's strange - why would somebody put a mark on a note that distinguished it from the real article, if they were trying to counterfeit it?

As you say, I think the stamp was put on after it was recalled.
And here is counterfeit note that don't have stamp.
https://en.numista.com/catalogue/photos/iran/67c88b2439a592.37392266-original.jpg

It's a different type(P-131b), but a genuine note is like this.

There is also counterfeit note with holes punched.