I thought Shakti started at k7, and k5 and k6 have also a Kx implementation.
BTW, https://ngn.bitbucket.io/k.html is a great overview of K implementations and which version they are implementing.
Yep, for a while Shakti was k7, but then Arthur started over & it became k9. Not sure what happened with k8
k5 was also known as "kOS" and was to be a version of k as a full OS that could run directly on hardware - I believe that got abandoned due to the pain of writing drivers
edit: k5, k6, k8 were apparently never released (https://aplwiki.com/wiki/K)