This is a genuine Honda lawnmower replacement part, p/n 08530-VA4-K10. It is used to tighten the handle joint. It is new old stock and fits many different models.
Note, included with this new knob is one well used one along with the corresponding bolt and washers. So if you lost one, these. Extra parts will get you back in business.
For questions, text or call preferred. This item is available if the ad is posted.