Years ago, Szell's M6 was considered one of the best recordings of the work, and many of the long-term veterans knowledgeable of the work still consider it one of the best. The one recording of approximately the same vintage that I think it sounds similar to is Kubelik's DG account. It may be a thin-sounding recording, but that can be overlooked for the performance. Incidentally, this was a recording made in-concert. Also, there is a Szell in-concert M9 that was released as part of a multi-CD Cleveland Orchestra contributors set years ago, and is considered excellent as well. And of course, there is the excellent M4 released commercially that most everyone knows about. It's a shame that Szell recorded no more Mahler symphonies than these three. At the least, I think a Szell M1 and M5 would have been good additions to his recorded discography.
Wade