This is the 4.2 (RTM) version of the .NET Micro Framework.
!!!!!PLEASE READ!
The SDK and PK documentation (.chm) files are not signed, so after you download the files you may need to unblock them before they will show content. You can do this by opening the file properties on each .chm file and pressing the 'Unblock' button at the bottom of the 'General' tab.