mas files get deposited into the highest version number of the car you want to use. for example, to use a skin for the Aston Martin GT3, you would place the mas file in the following folder: C:\Program Files (x86)\Steam\steamapps\common\rFactor 2\Installed\Vehicles\AstonMartin_Vantage_GT3_2019\3.59 All S397 cars have two folders, one is open(the higher odd number) and one is encrypted. (folder 3.58) So you place the mas file in the highest consecutive version number. Some modders create their own folder and the number can be much higher or even lower. So look for the two folders. 3.58 & 3.59, the mas file ALWAYS goes into the 2nd folder. Many 3rd party cars or tracks only use one folder, in this case, you merely place the mas file in the highest existing version. Also most, but not all, cars using the mas file can be extracted just to the rFactor 2 folder.(there should be some instructions with the skin) This works IF and ONLY IF the modder has included all the folders in the zip file that are needed to correctly place the mas file. Finally. Mas files are great and easy BUT, if the core car gets an update(remember folder 3.59?) if that is updated by S397 or a modder to a new version, the mas file will have to be moved from the old folder to the folder with the latest version number. (for example from 3.59 to 3.63)