The CB750 changed motorcycles overnight, it's the first "superbike", the first modern four - and the other Japanese makes scrambled to catch up. The Z1 is, arguably, superior to the CB750 but historically the Honda 750 is more significant.
The early 750 engine cases were sandcast. In itself this is hardly exceptional, but Honda changed to a diecast case production process when the numbers ordered spiked and made the extremely expensive dies economical. These eliminate cumbersome sand casting process steps and eliminate common casting failures from the fairly delicate sand forms and cores.
So the sandcast engines were obsoleted after diecast components were introduced. Several changes were made that are easy to see.
Thus the earliest examples of the world's first superbike are easily identified, and limited.
Due to a bad design decision, the early bikes were prone to chain failure... and another questionable design decision meant a chain failure usually would crack open the engine case. Many sandcast engines with this condition repaired under warranty had diecast case replacements, making them not quite sandcast although the engine numbers indicate they are... and dealers sometimes neglected to punch the number onto the case anyway.
So... these eminently collectible machines had a limited production run, many were crashed or scrapped, others had cases replaced with the later production diecast type.
The market value of an original, matching number (the engine # never exactly matches frame #, but with a believable close match) and correct other parts (many parts changed in early production and super anal experts will know exactly what parts your serial should have) is not related to any factor of performance... it will almost certainly never be ridden. The astronomical value is only as a rare and sought after collectible.
That said, the sandies did perform better than later models in some ways. Later models were detuned due to government rules for emissions, and for longevity... Honda wanted them to get out of warranty before breaking, and had not designed for them being run flat out all day on the USA's interstate highways.
