There is no specific "version key." A single legitimate product key (usually purchased via Steam or the official ETS2 website) unlocks the entire game.
Your key will automatically unlock every version from 1.31 to the most current release. Euro Truck Simulator 2 Version 1.35 1.31 Product Key
Keys found on sites like Scribd or public forums are usually blacklisted or already used. There is no specific "version key
Here are a few tips and tricks to help you get the most out of Euro Truck Simulator 2: Euro Truck Simulator 2 Version 1.35 1.31 Product Key
Version 1.31 of Euro Truck Simulator 2 is another significant update that brings several new features and improvements to the game. Some of the key features of this update include: