Don't know. It is the same front brake as the front brake used on the G85CS. Does that mean it is AMC?
Two Americans stuffed a Atlas 750 engine in a G85CS frame and raced it in the California desert. That is what the Norton P11 is based on. Stories vary, but that is the one I like. I'm not building P11 replicas and selling them, so my story may not be accurate.
The brake plate is not listed in my White Norton Villiers master parts list book for 1966-1967-1968 P11 and others.
This is what it looks like on the outside of the plate side. It is a 7" SLS brake.