I'm pretty sure it was s/n < 20000. On Changstar we did an analysis up to around that with six different pairs from low hundreds to 20000 s/n are some. Some very small variance - but in the end they were all HD800s. Some of the claims early on that later models were warmer or less bright are likely bullshit and have more to do with different chains, using more synergistic amps and DACs, or amps with higher Z out. The industry did change a bit with respect to components because of the HD800. The very late HD800s > 30000 s/n could have been different. I have never touched one of these.