The 1978 Honda CB400, and many, many other motorcycles have what’s called a “cush drive.”
Royal Enfield introduced the cush drive in 1912.
The way it works is that the rather than just bolting the rear sprocket to the wheel, the sprocket fits into a set of rubber dampeners. The dampeners are meant to reduce stress on the gear and wheel when force is applied when accelerating and decelerating.
Obviously the rubber dampeners don’t last forever. So if you’re planning to replace your chain and sprocket, I recommend getting new rubbers, if they’re old. For my CB400, it cost me about $75 for a set of four.
When determining to replace your cush drive dampeners, don’t base it on your sprocket seeming to be “loose.” The sprocket will have some play in a cush drive. Base your decision on how supple the rubber is and if they appear to be banged up. My rubbers weren’t too bad, but there was some stress marks on them and the rubber was feeling dry. I would replace them all as a set.