As far as I know, a prototype is a knife that may not see the light of day as any sort of available folder; It's just an experiment. A pre-production piece is something that WILL see production machining and release, and the actual pre-production version is something used to represent the final version (or close to it).
Robert Joseph Ansbro
If it can be written, or thought, it can be filmed.
-Stanley Kubrick, 1928-1999