Windows 1.0 Development Release 1

Windows 1.0 Development Release 1 was the first in a series of Development Releases of Windows 1.0, which were distributed to application developers throughout 1984.

The first of these was probably compiled some time in 1984-05, possibly on 1984-05-29 if the date in DR5's  is indeed a leftover from this build. An article in the 1984-05-07 issue of InfoWorld magazine claims the first version for developers would be released towards the end of 1984-05, further supporting the theory that the Development Release stage started around 1985-05.