Not sure if you heard my talk about this on the call yesterday. Here's the thinking:
504.08: lowest temperature AAB for reader stability, but also lowest flying. Not designed for dual RPM capability so it doesn't have much margin at 5900 RPM - which is a huge volume program. Timing is that we have the engineering build nearly through reli and the qual build is just entering reli testing.
504.18: Higher flying version of the 504.08, trades off reader temperature for dual RPM margin. HGA builds this week. No drive build planned.
504.20: Designed for dual RPM capability, trades off humidity sensitivity to flatten out the clearance profile. We are just in the process of getting the masks ordered for this design.