CB750 got improvements all the time during years to come.
Just see the engine upgrades. Side covers that K1 got also for a reason. But the collectors market is different.
The very first version is something special. Memories about the bike's impact another one.
It is not strange that Z1-900 is a collectors choice. The bike's design is very good. Tank, side covers, rear part with rear lamp together with seat an almost perfect combination. Gauges look good too. On top of that the paint that match it all really nice!!
The fork's rake look a little bit different for me though.
They should have sold more of it, like CB750, more bikes and parts for sale at a lower price today.
I'm old enough to have had my chances to buy one for decades ago, but did not.
I can wish to meet a person with a good but not running Z1-900 that want to trade it for a good running CB750 in good shape