I'm guessing it's a 5x7 with a reducing back, but it's kind of hard to tell from the photographs. I measured my Whole Plate 2D and the outside dimensions of the back is approximately 10 1/16" x 10 1/6" , and my 5x7 2D is approximately 8 3/8" x 8 3/8" . Hope this helps.

Roger


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