Windows Neptune build 5067.1

One file, namely, with version number 5.50.5067.1 and product name Microsoft(R) Neptune (TM) Operating System is found in Windows Millennium build 2380.2, but it is unknown whether this Neptune build ever existed.

Some speculate it would probably be part of the Milestone 1 stage. Judging by the file dates, it would have been compiled before 1999-09-20.