'18 and '21 HRC air boxes are different. The '21 air box fits quite tight against the frame. It's also got a very high back with 2 holes cut into it.

A '21 also has two cut outs on either side of the box on the top edges where the F3 carb stays can be fitted.

The picture in the first post is a MC21 unit.
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If I have to take the carbs off once more...