ADHESIVE PICKUP, essential for book repair
This is not an eraser, but a specialised crepe adehsive pickup for use in removal of adhesive residue in book and paper repair. Unlike some square shaped ones, these are flexible and very handy to hold and use. You will likely never need more than one - they last a very very long time.
For professional binders and conservators, it is also worth noting that these pickups easily restore to a softer feel after holding in your warm hands for a while. Some crepes won't do that.
Affectionately known as the 'fish finger' in the trade, this is an invaluable book and paper repair tool.