The idea behind Honda's little pressed-steel "guard" is more of a "guide". It is supposed to keep the chain on the sprocket long enough to throw it out behind the bike if the chain breaks. If the chain is properly lubed and has no kinked links (from galling or rust lock), then it works: otherwise it just crumples up from the chain and helps to crack the engine cases, anyway.
In the K0 sandcast series, the sprockets were 16/45 teeth. This proved too small for the 530 chains of the day, and many broke from American abuse (it was interesting to note that almost no such incidents were reported in the rest of the world at the time!). The first "fix kit" was a 17T front sprocket with a 48T rear, and with the little guide to which you refer, in the hope of sending the broken chain out the back, and an endless chain that required removal of the swingarm to change out. The next step was to replace the top clutch plate with a slanted-cut-cork style plate to induce a little slip when burnouts were attempted. This clutch mod went away before the K1 appeared, though, as the single plate wore quickly from all the slip.
The "final solution" was to use the 18T countersprocket and 48T rear, with the 530 endless chain of 100 links, and the rear sprocket was changed to have tapered teeth and a base circle of 0.5mm larger diameter than ASME specs. This proved to make VERY long-lasting chain and sprocket life, but it overgeared the 750 and reduced top speeds by almost 15 MPH.
It was this history of chain breakage that made Honda use the 630 chain on the later "F" bikes, with their added horsepower.
