Windows NT 3.1 PDK 1 IDW 15 build

Windows NT 3.1 PDK 1 IDW 15 is a build of Windows NT 3.1, which was mentioned in the file  from the Windows NT 3.5 build 782.1 source code. This build was being tested on 1 October 1991, so it was almost certainly compiled shortly before that.

The National Museum of American History has a disc containing this build, and posted a picture of the CD.