Perforated rectangular box. A reel of light sensitive paper would have been placed in the inside well and threaded under the well-staple and through the slit in the brass catch. This path takes it under the light window. By pulling the end of the tape a fresh area of paper would be exposed. This would fade and could be compared with the four samples surrounding it at the window.
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Dimensions:0 - Whole:W 3cm (W 1 3/16")
Dimensions:0 - Whole:D 3.5cm (D 1 3/8")