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Leon:

I believe that SGC typically goes with an "Authentic" grade on any item that does not have catalogued/standard-sized measurements as they would have nothing to compare the item to. I have a number of cabinet cards and the like and they are all basically "Authentics". The Peck & Snyder Cincy may be an exception as they have handled a number of those and could have a very good idea what the measurements should be.