Partway into the CB550's production, a change was made to the rockerbox or cam cover. The rockerarm shafts from engine number xxxxxx (see service bulletin xxxxxx) are pinned in place to prevent their rotation. Why was this done? Is it worth it to put the later rockerbox on an earlier 500 or 550?

I think if you have an opportunity to fit the later rockerbox, that is, if it is not too much expense or trouble, then yes you should do it. Fortunately, the used parts market for the 500/550 is huge, even better than that for the 750. So it shouldn't be hard to find the part. But if you do not do this, nothing bad will happen. The shafts may rotate a bit (though I never witnessed it in my 500) but this will not trash the rockerbox casting or hurt anything else. The situation is no different from the fit of the piston and piston pin, another "floating" arrangement between three mating parts.

I see on the sohc4.net forum they are now aware of the classic Honda mechanic's trick of preloading the shafts with an o-ring. They dismiss it as an inconsequential, shadetree fix. However, that is a slur and a misrepresentation. The o-ring trick works very well. Honda dealer mechanics of the period used this solution every day. My own CB500 I sold with over 92,000 miles on after benefitting from this technique the whole time, and my current one has it also. Never any problems. Don't let anyone tell you that you must make the changeover to the later rockerbox. It's just not so, and simply one more example of extemporaneous, incompetent forum drivel.