This is very affordable, unmounted, 1 inch (25.4mm) diameter optical slit, made of aluminium.
The front side is marked with slit length and width, the back side is left blank. The thin middle line marked on the front side helps with the alignment.
Length: 12mm
Width: 400um
Width tolerance: +0/-5um
Thickness: 0.2mm (standard)
Great for DIY spectrometers, slit-scan imaging, image- and laser beam sampling or other optical experiments.