http://www.jonmillen.com/CurtaGL
Jon reports that the design and measurements are taken from the plans on VCalc.net, for a Type I, ver 2.0.1. The model was reportedly created in Blender and animated in Unity, and runs on recent browsers via WebGL. It's lots of fun -- check it out!
This version models the exterior only, but the moving parts move, and it does do the numbers!" For best info, see the instructions!