This is very affordable, unmounted, 1 inch (25.4mm) diameter optical slit, made of aluminium (0.2mm, 0.1mm thickness) or stainless steel (0.05mm thickness).
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.
Stainless steel slits have no marking on either side to reduce thermal deformation of the thin material.
Width tolerance: +0/-5um
Thickness: 0.2mm (standard) or 0.1mm (special) or 0.05mm (special)
If required, the area where the slit is located can be further thinned out by laser etching down to ca. 0.1mm thickness. This is in most applications not required but can be done for a higher price (see options next to product photo).
The 0.1mm version is a 0.2mm thick substrate which is thinned out in the middle area by laser etching.