Yes I remember that we met each other at Ilford.
I hope that I can explain what the differences in use are of the covers.
In the picture you see the 2 probes.
Left is the clear one (151) for spot measuring. In this case I fitted already the ∅ 4mm (106) stop. Without this stop ∅ is 6mm.
Right is the translucent cover for semi-integrated and integrated readings. This is the 104. This one is used for metering a part of the pictures where color and density are fairly neutral, no dominances in color. When in doubt, it is advisable to measure multiple points and then averaging.
The diffusion material has to be fitted under the lens and is used for fully integrated readings in combination with the 151 with 6mm and of course fitted with the 152. Measuring in the middle of the baseboard.
Hope this helps.