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-
and also information about one build in 78XX series is available- 6.2.7891.X.XXXXXX-XXXX
The last build whose info was available was Build 7904 which was compiled on 16th December 2010. It would be interesting to note when the Microsoft comes out during CES 2011.