This was from a working university physics laboratory. The slit is adjustable via knurled knob. it's graduated to 8 mm in 100 um micron divisions. the knurl runs freely. It's a used fully working item.

For international sales outside of the UK. customs and duty is the responsibility of the buyer.

returns accepted.

I have another one available for a cheaper price which has not been listed yet. The other one doesn't have the blackout disc at the back or the screw used to block the service hole.