One of the main engineers for the sk19 or something like that up and quit, started making their own 'better' version (full auto) of the sk19 which is the rattler. Of course the sk19 was also someone's 'better' (removable magazine) version of the k1. About the time the rattler came out I think fx (hmm now read as donnyfl) bought remaining sk19s or so the story goes, and they made a sk19 with a fx liner system in it, which is of course their spin of the gun.
At least thats about what I remember. I currently still have a older sk19 in 22. I am very curious as to how well the fx liner in the sk19 shoots.
Allen