Wheels Manufacturing 7 Speed Cassette spacer - 4.5 mm

This spacer, sold individually, is the right size to allow you to use a 7-speed cassette on a freehub for 8/9/10-speed cassettes. 11-speed MTB is the same, 11-speed road is not. Sorry, that's confusing and is Shimano's fault.

It's a pretty niche use case, but sometimes you've got some nice 7-speed parts you want to keep using on other wheels!