It's not binding on the body of the block but at the top. You might try putting some Dykem on the flag, or marking it with a black Sharpie then running it in the action to see where the contact point is. It's most likely rubbing on the top edge of the flag that faces the hammer. (the top edge facing to the right in your photo)
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Ret. LE, FA Instr, S&W Armorer
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