insert Letter [解決]

What these it mean “insert Letter”? where I can see it?

Under the serial numbers on the under print, no idea why the word insert was chosen though.

thank you, you mean its part of the serial number?

No under it, the serial is printed over it. Just look at the obverse pictures of the example note were the serials are, on the underprint of note is an A (I can't edit a picture right now to show directly).
 

 From that link, this is the note 

 and behind the serial numbers is a large letter, it is A on this 

note shown - others have different letters, which I added over the top in green. 

thank you. now I can see.