A "first printing" 70mm of Empire would be worthy of preservation because it has some differences (for example, some extra Rebel fleet establishing shots were added for the general 35mm version). The problem is that any original prints would be on the extremely fade-prone pre-LPP 70mm stock. (They would look more like the film cells of Jedi than the ones of Empire - which I theorize to have come from a print struck for a reissue, since they are LESS faded than the Jedi cells, and at least one cell has turned up on eBay showing Luke on the weathervane with the Falcon dish composited in - a change made after the first 70mm run.)