Doh, yes, system 6 was current for quite a while. I wonder what other thing from that era I confused it with. I swear something around that time skipped a 6.
It seems Unix vendors really don't like low version numbers. Of course, Windows famously jumped from 3.x to 95, so it's not like they're any better.
Microsoft was surprisingly good at not skipping numbers, until the jump from Windows 8.1 to Windows 10, which also lead to the NT kernel jumping from 6.x to 10.
A lot of programs refused to work if there was a `9` in the version name.
That was of course because those programs needed something newer than Windows 95 or Windows 98. So they just did a string search for `9` in the version name.
There may be a few programs that wouldn't work on `8.9` or `10.9` if they were released for the same reason.