One file (
icmcui.dll) with version number 5.50.5117.1 is found in several Windows Millennium builds (4.90.2470 to 4.90.3000), but it is unknown whether this Neptune build ever existed. This could be the last Neptune build ever (from Milestone 2 like builds 5111.1 and 5116.1), which would likely be compiled someday before Christmas 1999.
Whistler bug reporting page
Version 5.00.00.5117 is also mentioned in Bug reporting page versions in Whistler builds 2211.main.000309-1512 and 2223.main.000411-2307. The later is the last Whistler build that dumps bug reports into
C:\neptune_bugs, which suggests that the bug reporting page came directly from a Neptune build.