version 3.1.3 refers to the version of the operating system (iOS) you are running not the iPod generation. You probably have a second gen iPod.
Here is a link to show the case differences http://support.apple.com/kb/ht1353.
The main difference is at the back where the 3rd gen has more details etched on the bottom of the case.
Thanks Milvus the link you gave opens but only to a blank page. There appears to be etching on near the bottom on the back but it's so faint & tiny that I can't make it out