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DavidBill's on the money. The images are reprints of the T5 photographs. I saw one postmarked 1915 once. They are sometimes blank backed, and sometimes with generic scorecards on the back.
Though they are not popular, and nowhere nearing the value of the T5 Pinkerton Cabinet Cards, they are legintimate, vintage and scarce. I don't think I've ever seen a Mathewson example before.