Not arguing with Ron's post, but it wasn't just poor maint that caused hundreds of chain breakages. Honda had a real problem with chains back in the 1970's, the metallurgy simply wasn't up to the power that CB750's were producing, that was why they finally went from a 530 to a massive 630H chain for the CB750F2/3 etc, because the chains of the era just couldn't handle the power.
I've got a mate who bought his shiny blue K2 brand new (and still has it) he never raced it, and was absolutely fastidious regarding maintenance (the bike did well over 100,000 touring/commuting miles without a rebuild) but his chain broke, just the same. Modern, good quality 530 chain can handle 180 BHP plus in bikes like suzuki Hayabusa's, but an OEM CB750 chain is only good for showbikes, and beating your red-headed step son.......... 
My poor maintenance comment was weak. While that could contribute, there was definitely something else going on. Certainly bikes with impeccable maintenance were breaking chains. You know, I was working at Honda Shops when the CB750 came out. At Burke's Honda in Ft Lauderdale, for the 5 months I was there (irresponsible hippie type) we must have warrantied 25 broken cases. Everybody was freaking out, from the shop owners to the district reps to American Honda. The owners were pissed, but they all liked in a dark sort of way, the argument that it was just too much horsepower for the chains. Maybe, we'll never know for sure. But that doesn't square with all the H1/H2s, and hopped up Sportsters, and drag racers that weren't breaking chains.
Certainly Honda was happy to lean on that as a reason and many of the service bulletins referred to using this chain over that one, converting to rivet links, changing sprockets, on and on. etc. It was so much that in my feeble mind I thought they were grasping at straws. They had to do something.
As a shop flunkie, I was involved in setting up the bikes which included juicing up the batteries and running the vent tubes. On the sidestand, the end of the tube could hang just over the chain, and condensed acid could drip, even a drop would cause a weak spot on the chain. This came out in a Honda Bulletin years later. This story makes much more sense to me.
But as i mention, it was mystery theatre, no one knew for an absolute fact what was going on. Little by little the problem went away. Maybe it was less horsepower and better chains. Maybe it was the quiet supply of a longer battery vent tube.
The service bulletin #98 came out in June 73; "Batt Vent Hose and Drive Chain" was the best kept secret of the time. Unfortunatley I can't get the text, but we've had this discussion before and someone posted it.
So its not a discussion that can be concluded in my mind. I'll accept anyones idea that it was the chains. I can'tprove otherwise. I prefer to lean on the battery hose . AS many bikes of the time had the horspower to break chains if that were the case.
Back to the subject of someone making a guard plate, at this point in time, its not necessary.
