The 23w bulbs are only suitable for the Artograph Tracer.
For anyone with the Superprism or Designmaster, Our supplier no longer supplies these bulbs. You can still locate them from electrical stores on the internet. Search for "Single Ended Halogen 250w 240v E27/ES Light Bulb"
A good comparable price with other sellers Brian Thompson (Thornton-Cleveleys, United Kingdom) on 17 Jul 2014
Good Oyetade Durojaiye (Bristol,, United Kingdom) on 4 Oct 2017