Bi-Directional 40mm-50mm Fan Adapter
These two sizes are confused frequently enough that it warrants an adapter. I think it's because the 50mm fans have 40mm mounting hole spacing.
I don't have a 40mm fan here at the moment, so I'm not 100% sure about the hole spacing, but I took measurements off of a printed part made for 40mm fans, so it should be very close. Please comment if you find it does not match, and I will make the corrections.