ADHESIVE PICKUP, essential for book repair
This is not an eraser, but a specialized crepe adhesive 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 soft feel after holding in your warm hands for a while, or sitting on it for a bit :) Some crepes won't do that. Despite some online advice, please do not heat in the microwave or in hot water, as this makes them even more rigid over time.
Affectionately known as the 'fish finger' in the trade, this is an invaluable book and paper repair tool.