[Electronic M]

If you argue a prototype as being a set designed before the [NTSC] standard, then the CT-100 could be considered a prototype too. Sure it had some minor circuit and cabinet changes, but it is pretty much a model 5 prototype pushed into mass-production. ...Even mass-production is not a good delineation between a prototype and production model since many later consumer model sets (even 21"ers) were made in such limited quantities that production methods often were the same as small run demonstration prototypes like the model 5.

IMO the true delineation between prototype and consumer set is in whether the company intentionally sold them to civilians [members of the general public not working for the TV industry] through their dealers. Every model has a prototype...what makes a set consumer is when someone brings the prototype to production as-is and says 'make X more just like this and add them to our current consumer offerings'.