A spring catch in the frame locates the plate holder in the correct position. There are two pairs of positions marked on the dark-slide: one is for two exposures 3 ¼" x 4 ¾" on a whole-plate; the other is probably for exposures on smaller individual plates held in a reducing frame in the plate holder and with a mask over the aperture in the frame. There is a clip on the frame possibly for this purpose.