Sorry hka I have neither a digital camera nor the skills to operate a very basic scanner that my son left behind but here's a description of the mystery cover.

Imagine your diffused cover but with a slightly darker but still smooth surface on top. Then instead of a rough surface underneath which is like many small pyramids, it has a smooth surface and over that lower surface is a white plastic cover which ressembles a round tent like the kind you get in a circus. However the top of the "tent" has been cut off to make a round hole which is about 17mm across.

So when this cover is on the probe the light from the neg strikes the smooth top of the cover but can only penetrate to the measuring cell through the 17mm hole.It cannot penetrate the white plastic which is opaque

If you took off the white translucent diffused cover on top then you would have a white opaque plastic bowl shape with a hole in the bottom. It is a bowl with a hole in the bottome when looked at from the top and a circus tent with a hole in the top when you turn the cover upside down

This is the best I can manage as a description. This 17mm hole in the plastic bowl sits directly onto the measuring cell.

I hope that helps you get a picture of it