I was a Honda mechanic at Pasadena Honda when the Sandcasts were introduced and got to see all the recall mods as they were applied. I know, I'm old. The front sprocket guide was used in theory to prevent a "broken" chain from bunching up and going through the back of the cases. They were made available in 1970 along with the revised, shock reducing rear wheel dampers. Sometimes they worked and sometimes they did not. I have seen both the chain AND the guide go through the cases. The camshaft profile change and a few other minor engine mods in 1970 reduced horsepower enough that along with stronger, riveted master link drive chains, the problem was minimized.
As with all previous Honda models, the early Sandcast chains had a master link with a removeable clip so you could "open" the chain to remove the rear wheel. The removeable clip was supposed to be installed in one direction per chain rotation. The clip could be slung off if the chain side plates stretched longer than the clip could accomodate or if the removeable clip was installed backwards. Many of the "broken chains" were not actually broken but rather the master link would fall out.
The guide on the rear sprocket was implemented to accomodate removal of the rear wheel with the "riveted" and subsequent "endless" chains. Instructions were to loosen the axel, spin the chain tensioners down, push the rear wheel all the way forward, move the chain from the sprocket to the guide, and then pull the rear wheel out. The rear sprocket guide was used to hold the endless chain from getting in the way with enough slack in the chain to get the wheel out. Reverse the process to reinstall.
Keith
