According to Canouna, one of the important source for Windows 8 builds have stated that build 79XX was a part of Milestone 2 and build 78XX was a part of Windows 8 Milestone 1. Here is what he posted on Russian site winreview.ru -
Windows 7 Post-RTM: 6.1.77xx.0.winmain.xxxxxx-xxxx Windows 8 Milestone 1: ?6.2.78xx.0.winmain_win8m1.xxxxxx-xxxx Windows 8 Milestone 2: 6.2.79xx.0.winmain_win8m2.xxxxxx-xxxx
We have already posted a couple of builds in 79XX series-
Stephen of UX Evangelist has reported that Windows 8 Milestone 1 has been compiled.it seems that Microsoft has already started working on Windows 8, Build Number is Build 7504.0.amdfre.winmain.090528-xxxx ,though it’s still to confirm whether the build is legit or not.It is to note that the earlier builds of Windows has build number of the previous OS so they would represent Windows 8 as Windows 7 in earlier stages.