The [CB750]22nnnnn series numbers indicated the K3, and they lost the "beeper kill" button on the left handlebar when compared to the K2, and got different-looking instruments - and the 'RUN-OFF' switch's lever got moved to a right-angle version instead of being inline with the pointer (not to mention 3 different carb calibrations, 2 different exhaust pipe innards and 2 different spark advancers in one model/year, the K3).
HOWEVER....since there were also 2 factories making these bikes after the K1 (approximately) 1/71 builds and until the K4 version, the numbers between about 4/71 and 9/72 got a little "mixed up" sometimes. This came about from bikes that were partially built but got new (or old) "kaizen" version parts, like differing engine pieces inside or electrical harnesses, because the demand outstripped Honda's ability to fill the orders for the bikes. Usually the more visible parts (tank, side covers, emblems, instruments) stayed very close to the "K" version that matched the title for the bike - that is, until American Honda started letting some of the larger dealers 'retitle' unsold bikes over into the next year, thoroughly confusing the "K model vs. year model" situation. This latter practice happened for sure in Illinois, California, New York, Indiana, North & South Carolina, and Arizona - but not everywhere, and this list might be incomplete, as differing State's Motor Vehicle Departments treated this practice differently.