iMac - Can't install a Supplemental Update (10.8.5)
As the dates on the pages for both the Combo &amp; Supplemental updates are identical, the chances are the Supplemental may have been later rolled into the Combo. The Supplemental was released on 3 Oct 2013, which is the last modified date on both pages.

Check About this Mac > system report > software [names may be slightly different, I've not seen 10.8 in a while]
10.8.5 should show build number 12F37, the Supplemental 12F45 in brackets after the System Version.

Initial research done using Wikipedia : OS X Mountain Lion

After comments &amp; further research, Build 12F25260 is a specific build for iMacs with NVidia GPUs &amp; does include the Supplemental update.

Refs :  



https://discussions.apple.com/thread/7292272?start=0&amp;tstart=0
http://www.xlr8yourmac.com/archives/aug15/081715.html
http://www.nvidia.co.uk/download/driverResults.aspx/89564/en-uk