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Background
During the quest
Burlap is the New Stripe, you'll find yourself in the Toussaint Prison. While you're in the main courtyard of the prison, if you turn on your witcher senses, then you'll detect a loose brick on the eastern side (#1). Behind the brick you'll discover Notes Found in the Toussaint Prison. Reading the notes will trigger the quest.
Walkthrough
From the notes, you'll learn that Gaspar is in need of a
Small Hammer and Chisel for Carving Stone, and that if you can find such an item for him and deposit it at "the drop-off spot" (aka behind the loose brick), then you'll get a nice reward. Conveniently, you'll discover a
Small Hammer and Chisel for Carving Stone in the cart in the northern part of the courtyard (#2). If you place the item behind the loose brick and wait for a week -- by meditating, since you can't do much else in the prison -- then when you check the brick again, you'll find a
Prison Poster, which you can use as a decoration at Corvo Bianco. You'll also receive 50 xp for completing the quest.
1 - Loose Brick
2 - Cart