The sheetmetal plate is mostly there to try to "catch" the chain if it falls off the rear sprocket, keeping it from locking the rear wheel suddenly. This happened a couple of times on the 1969 K0 in the beginning, which led to this addition in late 1969.
The sprockets: the original K0 setup was the 16T front, 45T rear. This used a 98-link chain. It proved to be it own undoing as it wore so fast (with average rider's lack of chain maintenance) that Honda was faced with many chain replacements under Warranty, which was only 90 days long back then(!). It also led to many broken chains and crankcases. By late 1969 Honda had a retrofit kit out that consisted of a continous, non-masterlinked 100-link chain, a [heavy] sheetmetal chain guide that bolted to the crankcase right above the rear axle and surrounded the new sprocket, and a list of installation instructions that included removing the rear wheel and swingarm to install the chain(!). This method remained in production until the end of the K1 in July 1971, too, so the burgoening market for chain breakers really took off.
The K1 came with the 17-tooth front sprocket and 48-tooth rear sprocket for the lowest gearing found on the production CB750 bikes. This yielded a higher top end (more than 125 indicated, probably 120 MPH real), too, as the 750 could/can rev it in top gear on flat roads (I was there many times with my own K1, reaching indicated 130 more than once!). It made the chains last a little longer than the original setup, but chain manintenance had to be intense or bad things still happened.
At the end of K1 production (Fall of 1971) the 18T front sprocket was used with the 48T rear, instead. This slightly overgeared the bike as far as reaching top-end potetial is concerned, but improved gas mileage and vastly improved chain life because this sprocket, and its 48T rear mate, were specially re-designed. They had a base circle (i.e., the bottom of the chain roller's site) that was 1mm (some Honda bulletins said it was 0.5mm) larger than that required by ANSI standards, which at that time were the worldwide standards used for making chains. Diamond Chain (USA) had been specially contracted by Harley-Davidson, in 1970, to create a special new chain in size 530 for the new 1000cc Sportster they were creating: it had lots more torque than the old Sportster and Harley-D wanted a better chain. This became the famous Diamond XL chain, both unique and expensive, but perfectly engineered for bikes. It was immediately successful, even as H-D was falling into oblivion financially and AMF bought them. Honda bought 51% of AMF in 1971 (the Japanese were buying lots of Americana in those days) and immediately had RK of Japan [try to] copy the chain, as it performed amazingly well on the CB750 in testing, besting anything Japan had made. Those copies became Honda's standard chain for all their bikes, later also Kawi and Suzy as well (Yamaha stubbornly stuck with their older, weaker ones). The details of this design are outlined in my book.
That series of events changed EVERYTHING in motorcycle maintenance. The 750 had already changed the rider's image, but the maintenance had to be intense to ride it as much as it likes to be ridden: this new pairing of (undercut + short-toothed) sprockets and the XL chain made the 750 maintenance like a 350 again, and away we went!
Now for the details: the early 750 rear wheel hub had equal-sized rubber cushions for the torque blades. These wore out FAST, usually by 5000 miles, making for lots of chain lurch, snatch, and noise, not to mention damaged chains (and cases). Honda changed them to the ones you see now, with the thicker ones being on the 'pull' side and the thinner ones on the 'coast' side of the torque blades. This then became standard on all new bikes (at least, from Honda) with the 3-piece rear wheel setup like this one. They work well until they get so slack that the hub can loosely wiggle back-and-forth with hand force, then they get noisy and cause chain snatch, which is both annoying and hard on chains as it snaps the lube right out of the links when the chain whips like that.
For sprocket tooth choices: if you don't plan on taking long interstate trips, you can use the 17T front sprocket with either the 45T or 48T rear sprocket and get stronger throttle response in the lower gears, and pull 30 MPH in 3rd gear steadily with no issues. If you use the 18T front sprocket with either of these rears, you will want to stay in 2nd gear more often below 40 MPH in order to stay 'on cam' with crisp throttle response. Honda settled on the 18T/48T version in the K2 series and later bikes: with today's taller metric tires this means lower gears (2nd) in traffic for many riders as it acts like having about a 42-tooth rear sprocket.
Personally: I use the 18T/48T setup for most things unless I am planning on a lot of interstate riding. Here in the American West this means 80+ MPH legal speeds, and running 85-90 steadily is common, so then I use the 19T front sprocket with the 48T rear. Having the larger rear sprocket reduces the loss of chain lube at the rear because of the larger (albeit slight) diameter and slower change-of-direction of the rollers when they enter the rear chain. This was Honda's surmise, too.
At hiway speeds I have found that running the 17T front sprocket means lubing the chain at almost every gas fillup, while using the 19T front needs chain spray only once in a whole day, maybe two, of riding. It actually makes that much difference at the front sprocket! But, using a 19T front sprocket is not a drop-in affair: you must first trim the rubber bumper on the post inside the chain cover at the engine to clear the teeth and also trim the tips of the sprocket teeth back 1.5mm (do this with ALL modern sprockets, for reasons outlined in my book) so as to let the chain settle quietly and not bang up the rollers so much. This smoothes out the overall ride, too. I also trim the rear sprocket teeth at least 1mm, usually 1.5mm, too, since we cannot get the undercut-base-circle sprockets at the rear any more. Sad. I still have one rear OEM sprocket left: it is my last one!
