Terror-Flex would be MX model, The Big-un is only in 18",your KX is 19". (unless Prev Owner swapped out 18" rear wheel)
I had TerraFlex on my CR and loved it. I could really feel the weight slogging around at higher speeds, that was not a comfy feeling. I would like to go 18 myself and TerraFlex. That was the only tire that the 500 didn't destroy, (for Me).
Next tire change I'm gonna have balanced.
I still have a Kings Tire something that came with mine when I bought it, so I'll be looking for a new rear soon also.
Tuck\o/