I don't mean hardware prototype, but maybe a "business arrangement prototype".
During my web surfing I realised that the Nokia N1 tablet, released in 2014 with the innovative Z-launcher, was made by Foxconn under licence from Nokia, using designs by Nokia Technologies. This is not a secret, I just learnt (or re-learnt!) it. Lots of people on this forum are still looking for support for the Nokia N1 which appears to have been abandoned by both Nokia and Foxconn, but that's a different story.
The new Nokia smartphones are made by a partnership of Nokia, HMD Global and FIH Mobile (part of Foxconn), but in this case Nokia doesn't get involved in the design, unlike with the N1. Instead HMD and Foxconn do the designs together (AFAIK). This arrangement is very similar to the "prototype" arrangement of the Nokia N1.
So what did Nokia and Foxconn learn from the N1 experiment? I assume that what they learnt is that they needed a third partner to lead marketing, sales and some design aspects, which is why HMD Global was created. So far, it seems to be working very well!
I'm looking forward to seeing more great things from this tri-partnership in future. Maybe a tablet? ;)