I assume this was the regular practice. It sounds like an archive of all the work done.
I'm sifting through the production category for the first time and finding lots of interesting stuff! A few years late to the party on this topic, but -- yes, KGB is exactly correct. Records and Archives was just a vast storage area for specimens of all of ABNC's work. In fact, all the banknote companies had archives like this.
I am most familiar with this rubber stamp on stock and bond specimens, so it's interesting to see it on stamps. You will also find specimens stamped "Last Specimen, Reserve 0" -- meaning this was the last archival example, so put it back when you are done
! The implication is that there were often extra samples on file that salesmen might give to customers.
Here are a few specimen items with similar rubber stamps. A great amount of this kind of material came onto the market beginning in 1988, around the time ABNC was bought out by US Banknote Corp and slowly liquidated its archives in a series of auctions: https://www.coxrail.com/abn-archives.asp