Acorn OS

Acorn OS is a series of 2 unnamed operating systems that run on P1 and P2 iPhone prototypes respectively. It was originally thought that these were early versions of iOS. It is rumored that Tony Fadell designed the P1 OS and that Scott Forsal designed the P2 OS.

P1 "classic"
This ran on the P1 iPhone prototype and runs what appears to be a port of the iPod System Software to the iPhone, with a virtual set of buttons and a click wheel, but the iPod System software has a few additions such as a call menu and a diagnostics menu with stuff such as bluetooth testing and CPU clocking. A gallery of this build is https://sonnydickson.com/2017/01/06/the-ipod-based-interface-that-lost-out-to-ios-for-the-iphone/. Tony Fadell confirmed it's existence in a Tweet (now an X post)

P2 "system"
This ran on the P2 iPhone prototype and has a touch based interface with a bunch of crude blue boxy buttons on a white background with crude messages on the bottom. The gallery of this build is here https://sonnydickson.com/2017/01/11/how-apple-picked-what-came-to-be-the-iphone/

According to Bob Burrough in a youtube comment, this isn't iOS/iPhoneOS and his team designed it. According to him, iPhone OS was so secret that only an exclusive, select few people -- who had to be personally approved by Steve Jobs -- could see it. What you see in the video is a fake UI that everyone else -- hundreds of test engineers, carriers, etc -- got to see This likely applies to the P1 prototype as well.

Known footage

 * https://www.youtube.com/watch?v=P0LlVtlXEEA
 * https://www.youtube.com/watch?v=0eJZH-nkKP8
 * https://www.youtube.com/watch?v=vVT8RVWlPpA
 * https://www.youtube.com/watch?v=z-aXqyIukW0