This shirt kind of reminds me of the 60s or 70s, but it’s a plaid shirt that has a gingham feel to it, though the pattern is very large and more plaid-like. It has flapless breast pockets that do up with a button. It’s standard cut and does up with buttons.
In Stuck In the Middle (With You) (12x12), Sam wears it for the entire episode.
In Twigs & Twine & Tasha Banes (12x20), Sam wears this shirt when they meet up with Alicia and Max. He continues to wear it for the rest of the episode.
In Various and Sundry Villains (13x12), Sam wears this shirt from the beginning of the episode until they talk to Rowena in the Bunker before leaving to track down the stolen book.