It is possible this was a magic lantern slide. I pulled out a few of mine to check the dimensions. They measure 4 x 3¼ and the image area is about 2½ x 3½. that is fairly close to the measurements of your item.
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'Integrity is what you do when no one is looking'
"The man who can keep a secret may be wise, but he is not half as wise as the man with no secrets to keep”
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