You're right. I just downloaded old versions by myself and checked Details tab.
I'm not sure if the information below helps your situation but you might be able to guess the version you're using from "Data modified" information in the Details tab.
- ThingWorx-DotNet-SDK-5-0-1-103 : "Date modified" in the Details tab shows "2014/06/20 13:59"
- ThingWorx-DotNet-SDK-5-5 : "Date modified" in the Details tab shows "2015/03/31 16:47"
- DotNet-SDK-5-6-0-549 : "Date modified" in the Details tab shows "2015/09/17 23:24"
It appears that DotNet SDK newer than DotNet-SDK-5-6-0-549 has correct version information in Details tab.