Right clicked -> Properties -> Details tab
I see the following:
File Description: Newtonsoft Json.NET
Type: Application extension
File version: 126.96.36.199
Product name: Newtonsoft Json.NET
Product version: 188.8.131.52
Copyright: Copyright Newtonsoft 2008
Date modified: 9/17/2010 4:09 PM
Language: Language Neutral
Original filename: Newtonsoft.Json.dll
Doing the same thing on an older version of the dll gives me the same info except for a different modified date and size.
I know that the modified date is specific to my file os. I can't tell you which release the older version is because it doesn't say. It also says it is version 184.108.40.206 just like the new version. The size on the older version is 200 KB.