The actual revisioning does not get updated with this update, and to verify that the install was installed you can look at the fileset.txt.
The fileset.txt will be located in the installed directory similar to the following.
Here you can view the file in a text editor and the 13.1 update will have the imageBuildVersion equal to O.40e.2.1
Mon Apr 18 10:31:55 TZ 2011:: version=13.1
Mon Apr 18 10:31:55 TZ 2011:: imageBuildVersion=O.40e.2.1