yes, to mount that specific type or rear valence can only be mounted on Kouki rear bumper, just look at it closely in the picture: that is a kouki bumper and the valences jaunt out on the bumpers lip. Thus using the standard s14 USA-market (same as zenki) bumper it wouldnt line up and leave a space where the kouki bumper curves in and then jaunts out.
observe picture
A good rear valence for the stock 240sx/ zenki bumper is the OEM one u see sometimes on Aero package zenki s14 or Bomex (both look similar to me:
OEM
http://www.jspec.com/s14rearvalence.html
BOMEX
http://www.bomex.co.jp/item_img/20521320001.jpg