Years ago, 77-83 when we sold Yamaha's, I remember installing bar weights in one of our models but can't remember which it was or what year.
For some reason I keep thinking it was the XS400 model and it would have been around 78-79 perhaps later.
It consisted of a solid bar of steel about 6" long with o-rings on each end that was installed inside at each end.
Clutch side was already installed as the grip was already on, and just the throttle side needed installing because the bars were not mounted in the crates so as to keep the crate as narrow as possible for shipping.
Not sure how tight the fit was because the end would interfer with the throttle tube so it must have been tight, but really just guessing.
That was over 30 year ago. Many brain farts since then. That the best I can do.