Here's apparently a glimpse at the "mysterious" K6 action found in some recent Kurzweil stage pianos for example:

It looks similar to this photo of the Medeli SP-4200 action:

[Linked Image]

(Photo source:

So, it seems fair to speculate that the SP-4200 (and its rebranded variants) have the same "K6" action as e.g. the recently introduced Kurzweil SP6.

(Or, I'm completely wrong. ;))