A new RED plaid shirt for Sam! This shirt almost looks gingham from a distance and might actually be flannel. It’s got two breast pockets without flaps, but that do up with little black buttons. It’s a button up. (note: it was another fan who coined the name, but I do not know who.)
In O Brother Where Art Thou (11x09), Sam wears this in the dungeon of the bunker while guarding Rowena, and for the rest of the episode.
In The Devil in the Details (11x10), Sam continues to wear this shirt for the entire episode.
In Regarding Dean (12x11), Sam wears this shirt whenever he is not in a suit, which largely is when he is in the motel room towards the beginning and end of the episode, and when he goes after the witches and needs a bit of rescuing.
In All Along the Watchtower (12x23), Sam wears this shirt for the entire episode.
In Rising Son (13x02), Sam continues to wear this shirt until he’s able to get some sleep. (This happens after the argument between he and Dean, that causes Jack to leave the room, and Sam has to track him down and talk to him.)
In Beat the Devil (13x21), Sam is wearing this in his dream at the beginning of the episode.