I actually didn't know this. I thought that the reason it took so long to start the mods for 684 and lower was because of a backlog for new seats. I thaught they only reused the 75E seats and those are on the 75Hs. I have yet to be on a 75D without the new AVOD screens. I also don't belive the old 75X seats are slimline seats and I also didn't think they would work with the new High Density configuration on the 757.