I believe in the documentation still in someplaces and inadvertently in this blog I mention CE 5.01. This is the version of the OS that we shipped for Windows Mobile 5. Basically we shipped CE 5.0 (the general embedded flavor) in June of 2004, but the Windows Mobile team still had a long list of features that they wanted from the CoreOS team. So we spent many months focusing on this, making enough OS level changes that we justified having a minor version bump-up.
CE 5.01 is not something available to Platform Builder customers and isn't really doc'd on its own outside its existence as the core to WM5.
[Author: John Spaith]