Jonathan, so what do we call Amtrak cars prior to the head end power rebuilds? These cars did wear Amtrak phase One paint. HEP rebuild started in 1977 and took several years to complete. The 7+ year period of Amtrak rebranded passenger cars from private railroads pre-HEP are called what?
That is a great question and I really don't have a good answer for it. I have always referred to that era as Amtrak's transition era, or the steam heated era. I will need to do more research on that topic. I suppose technically they are all heritage cars, but the Heritage Fleet is a specific program of rebuilding.
One resource for cars that ended up in the Heritage Fleet is this wiki page.