mod_overrides, since texture/model mods go there and game changing things go in mods

Wrong. This is Add-On type of mask so it's not just a simple retexture but more like a game changing (additional content) mod. To put it simple, it goes to mods folder.

@3xBAR No. YOU are wrong. A custom mask is not gameplay changing and if you looked into the files you'd know that those are the files that are typically used for mods that add or change models that utilize the mod_overrides format. Also I've downloaded this and other masks, both went into mod_overrides.
tl;dr you're wrong and it goes into mod_overrides

OK, I get it now. We are both right and wrong at the same time, cause custom masks works the same way regardless where you put their files, mods or mod_overrides.
The more you know. Thanks for helping clearing that out. Cheers. =)

@3xBAR No problem and