Scaling Type:
Linear scaling (additive) for weapon upgrade stat bonus (+1 per copy)
Detailed Explanation
Gives you the option to get weapon upgrades with +1 stats. Meaning with 1 copy you can get 3 stat upgrade from leveling up a weapon a single time.
You can however get more anyway with a lucky shady guy. The items the shady guys sells are determined as soon as you enter the stage, and they can have an anvil. If you get an anvil from a chests, the shady guys that spawned in will still be able to sell you an extra copy, although you theoretically are over the limit.
Note that under normal conditions you can MAX get 1 anvil in the game
You can however get more anyway with a lucky shady guy. The items the shady guys sells are determined as soon as you enter the stage, and they can have an anvil. If you get an anvil from a chests, the shady guys that spawned in will still be able to sell you an extra copy, although you theoretically are over the limit.
How it Stacks
Each copy gives you the possibility of +1 stat on each weapon.