Last week, on Friday, Microsoft made a branch for the RC1 Milestone build. The branch is known as 'vista_rc1' and began with build 5504.16385.vista_rc1. Don't confuse this with the RC1 winmain branch that began with the build 5420.0.winmain.060427-1900 back in April, because that branch is still pressing on and will most likely see a build number jump soon enough. By the way, I should note that 5504.16385 is effectively equal to the build 5489.0.winmain which was built the same day.