It’s a dress shirt with a bold pattern of different shades of grey. The stripes on the pockets run on a diagonal. It does up with snaps. Although the dark horizontal stripes draw the eye – it actually has a very faint plaid pattern in the lightest shade of grey. Like most of Sam’s shirts, it’s cut in the Western-style.
Brand Information: Guess.
In Heart (2x17), Sam wears it when he and Dean are watching Madison to make sure she doesn’t turn after Dean’s killed Glen. He takes it off when he has sex with her (obviously), and then Madison ends up wearing it when she does turn and escape the apartment. Sam ends up shooting Madison while she is still wearing this shirt (presumably).
In Red Sky at Morning (3x06), Sam wears this shirt when he and Dean are doing research on ghost ships and Bella shows up to tell them about the hand of glory and recruit them for her thieving plan. He wears a tux for a bit, but after he changes out of it, he puts back on this shirt and leaves it on until he gets soaking wet in the graveyard rescuing Bella.
In I Know What You Did Last Summer (4x09), Sam wears it when he and Dean go to the church to find Anna. He takes it off immediately afterwards to stitch up the cut on his arm.
In Swap Meat (5x12), Sam’s body wears this shirt from when they talk with their former babysitter, until after Gary forces Dean to get a new hotel room. (Basically, he wears it for the first day of the episode).
FATE: In Heart, Sam shoots Madison in the heart with a silver bullet while she is wearing this shirt, and yet...it appears again. So, make of that what you will.
In I Know What You Did Last Summer (4x09), Sam jumps out of a window while wearing this shirt and gets a deep cut on his left bicep. Presumably, the only way to get that cut would be for the glass to have gone through the shirt as well. Unsurprisingly, the shirt makes a full recovery.