If you leave "The Bullet That Can Kill The Past" and have a "Ring Of The Resourceful Rat", you'll get an S item in return at the forge! : r/EnterTheGungeon
Five Artful Watches to Celebrate the Year of the Rat | Barron's
Is it Furbait? — is the resourceful rat from enter the gungeon...
Steam Community :: Screenshot :: WHY IS THIS GAME SO HARD (nice easter egg tho)
Enter The Gungeon: HOW TO BEAT THE RESOURCEFUL RAT - Beginners Guide and Tutorial - YouTube