Microsoft begin serving Windows 7 and Windows Server2008 R2 SP1 RC via Windows Update to Beta testers that opted to grab the bits for the first public release of the upgrade through WU earlier this year.
As you can see in the screenshot, KB976932 is being offered to both 32-bit and 64-bit versions of Win7 platform. Among the downloads offered for Win7 SP1 Beta were WUSignUpTool_x86.exe and WUSignUpTool_x64.exe. The two tools were designed to be executed on a Win7 RTM OS and change the registry so that the platform would signal to WU that the user wants SP1 Beta downloaded and installed.
"If you're already participating in the public beta via WU, go to Windows Update in Control Panel, uninstall the beta, select Check for Updates, then install release candidate. You don't need to run WUSignupTool a second time to be offered RC," Microsoft explained. "You don't need to install any updates before installing the RC."