AT20SL was the original in the series. The SLa had a slightly different body for the newly adopted 20 degree tracking angle. The SS had a beryllium cantilever in place of the original aluminum. All 20 series were merely hand-selected from the best of AT15 production (in other words, no real difference, just closest to spec).

In the real world, I doubt you'd find much audible difference between any of the series. I've owned quite a few (running an AT15E these days), and all are superb.