So what was the first GM concept car to get an XP designation?
From at least the mid-1940s to the early 1970s, General Motors used a system of XP labels to designate its concept cars and other design studies – a system that, at first glance, seems rather rational, but on deeper inspection was poorly or haphazardly implemented, to the point that we’re not even sure which was the first project to warrant a GM XP label.
Most automakers – at least in their early years, that is – follow pretty regular naming conventions. Henry Ford (on his third attempt to start a car company) started with the Model A in 1903 and made it all the way through to the Model T with just a few gaps here and there. Ferdinand Porsche’s Typ system was fairly well organized. So one would expect something similar from a company as complex from the outset as General Motors.
Read on here
Related – Mustang III – 1963 Ford Mustang Concept Car