Honda Service Bulletin #22, 8/28/70, "modified" chain cover 55 mm longer "installed free of charge on any CB750 manufactured prior to Frame Number CB750-1021880." There is no mention of any steel reinforcing bracket to go inside the guard. i'm pretty sure the "modified" guard was same as the short guard, i.e., same swing arm mounting from sandcast #1 up when diecast 21880 was fitted with longer guard.
Honda Parts Bulletin #750-35, dated August 28, 1970, "the following new parts are applied form Frame Number: CB750-1026845." The parts are pictured in the parts bulletin to include the bracket you refer to, pn 40512-300-000. Honda also call for the side plate on rear sprocket with the idea it will help keep the chain form derailing too far, should chain come off rear sprocket., pn 41211-300-000. Did the swing arm also change to accommodate fitting this metal bracket inside the guard and attaching it to the swing arm ?
Comical in a strange sort of way, at least from an attempt to redesign parts to prevent a loose chain from derailing and/or wadding into the rear crankcase wall, PB #750-36 (revised) calls for pn 11353-300-010, case protector and pn 11354-300-000, rubber chain guide. both these parts fit inside the redesigned 11351-300-040, cover, l. chain case. Honda by that time, was having to deal with by then a fairly significant number of warranty claims for broken crankcases. Factory was providing unstamped sandcast replacement cases to satisfy warranty claims. all because chain technology of the day could not manage the horsepower of an engine producing 68 hp, resulting in excessive and unexpected stretching of chains greater than recommended service intervals. and today we have stock out of the crate 180+ hp chain driven bikes and chain failure is generally not thought of.
