I suspect that information is erroneous.
Or maybe not depending on where you live.
AFAIK , the all important US market got every model of CB750 ever manufactured.
But over here in the UK, we jumped from the K2 straight to the K6, Honda dealers in the UK were kept supplied from a stockpile of K2's at a warehouse in Chiswick.
So in UK terms, the K2 ran from '72 to '75
No K3, K4, K5, or K8 bikes were ever officially sold here.
Same for the 750F series.
We missed out on the F0 and F3