The Microsoft OS/2 SDK includes pre-release builds of OS/2, beta development tools, sample code, and loads of documentation. These were released prior to the OS/2 1.0 and 1.1 releases. Microsoft charged $3,000 in 1987 for the SDK. It was criticized as overpriced, buggy, and slow.
These were superseded by the smaller and less expensive Microsoft OS/2 Programmer's Toolkit
Much more information can be found here: OS/2 Museum, OS/2 Beginnings
May 1988 - SDK 1.04
The 1.04 SDK adds the first preview version of the LAN Manager, but has no other OS changes.
Wanted: Microsoft OS/2 SDK 1.04 May 1988 disk dumps.