Yes Dave, the RCB was built on CB750 bottom end but had bespoke barrels and twin cam head that made the first iteration 16valve endurance racer.
That one had cams really close together as in this photo
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Then came the later RSC1000? (think that was it's designation) which was development of production 900 twin cam road engine that was the CB900 but a completely different engine to the SOHC.
That first post here seems to have grafted on the CB900 type head onto CB750 SOHC cases, which I've not seen before. So they appear to have made a hybrid out of production motors.
The brandy is not playing tricks