40, 41, and #420 chain all have the same pitch (1/2"). 41 and 420 chain have the same width (1/4"). 40 chain has a wider width (5/16").
This means that on a 41 or 420 sprocket, you can use 40, 41 or 420 chain.
A 40 sprocket would require a 40 chain, as the others are too narrow.
Here's a more detailed chart