My understanding is that an entire print was cut up into individual frames - the Jedi cells are faded, suggesting they were an original 1983 source (which is still odd - all 35mm prints of Jedi were on low-fade stock, did 70mm LPP not come until later?). Both Empire and Jedi have soundtracks, which would be unnecessary if they were struck just for cell purposes (note that the cells for the original film, which have the least fading, DON'T have a mag soundtrack, suggesting that they WERE from a new blowup made strictly for the cells).