I don't remember ever seeing the model numbers 5 through 8 being on the arms, would have been in the shipping box. as noted the difference between the 5, 6, and 7 were the arm cables supplied with them, ie, good, better, best. the 8 did have better internal wire, and shipped with the cables that came with the 7. I however don't remember any difference in tube diameter between them. possible the 8 had a thicker outer polyfin
covering...dunno. EM of the arms were all the same as far as I can remember. The 8 model would have been a lateral move with the Linn Akito in performance, with the advantage of the damping mech under the round cap on the pivot boss making it a bit more versatile in cart selection.