PMG has Frieberg # that is not in 21st Edition 0

[Dakota Metal Recovery]

This is my 1st post.

Found a 1969C $5 Minneapolis Federal Reserve Note Uncirculated with alignment cutting error.

Looking at PMG Population report they List 2 options for note  Fr. 1972-I  or Fr. 1972-Is

Can anybody tell me what would be the difference between an "I" or an "I s" ?

[Jennifer F.]

Good morning,

The "s" refers to the specimen version of this note.  

If you have any other questions, please let us know.  Thank you!

[Dakota Metal Recovery]

I was unaware that Fed was printing specimens in 1969. Maybe this should be graded. Thanks