Glass plate holder. Looks like the wire spring that fits across the slide near the felt trap..is missing. It holds that end of the glass plate in place..You pulled it back..inserted the plate and released the spring....
Got two of these type holders for 1/4 plate somewhere.
The glass plates fit into a slot at the bottom, then there's a sprung clip at the centre of the top. Those tabs push the plate up to the open edges, they negate any variations in plate thickness, which can vary between around 1.4mm to 1.8mm.
.....and the beauty is ..you can still buy ready to go sensitised glass plates from a chap in Serbia who makes them and also produces modern computer printed holders for them...but check he does your size (if you wanted any).
They are hand made plates .... bit expensive as viz 5x4 at 10 for £40!!
Have a look at https://www.etsy.com/uk/shop/ZebraDryPlates
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