Build 20110309_2315 would have been the original released build without any service packs applied.
When you did your install, it probably asked you if you wanted to install the latest service pack during the install options.
You can't uninstall a service pack, so you would need to uninstall and reinstall, making sure to expand the options for Revit during the install and selecting the option to not install the latest service pack.
You can find the curernt version number by clicking on the help dropdown (next to the question mark in the upper right corner) and choosing "About Autodesk Revit".
Joe