As far as I know, N-V never saw a "prototype" P11. It was invented by one of the people at Berliner, the US importers of various AMC machines. They were heavily involved in long-distance desert racing in California and found the 500cc AJS/Matchless dirt racers they sold were under-powered. Someone there tried installing the Atlas 750 engine in a Matchless frame. It was very successful.
While I was at N-V (Wolverhampton) in late 1967/early '68, a brand new P11 showed up with instructions to break it in and get it ready for testing. I did most of the break-in riding. I disliked the high level of vibration and poor handling. At speed, it would start to weave, getting to a displacement of almost a full width of an M-6 traffic lane at about 60 mph. My impression was that the frame couldn't handle the torque of the 750 engine. My follow-up rider said the weaving stopped once you got above 85 mph! I was riding quite a lot after dark, and I was amazed how frequently I had to replace light bulbs, particularly the headlight, due to the vibration. I also carried a small tool-kit to re-tighten fasteners that came loose during my rides. The tail/brake light came off on almost every ride until I bought some anti-vibration nuts to hold it on.
We discovered, a short time later, that a rider in the US had been killed in a desert race when his P11 got into a high-speed "tank-slapper" and threw him off. We were told to get ready for a UK "cause of death" lawsuit. Before I even finished the break-in rides, the lawsuit was dropped. I got the impression that nobody at NV at the time wanted anything to do with the P11. I was surprised, quite a while after I'd emigrated to Boeing, that N-V were actually producing P11s and selling them in the UK.
I rode the US built P11 for quite a while as my commuter, during the time I was breaking it in. It was by far the most popular bike I'd ever shown to my church's youth club!
