Does the Letus Extreme require an apochromat to mount on the JVC/Fuji 16x lens, or just an 82mm treaded ring?

The Letus Extreme should have an achromatic dioptre already screwed into the back of the flip (prism) enclosure.

I understand that there is choice of achromatic dioptres. Some serve across camera brands and models.

The 82mm ring does not itself contain the dioptre but is a short tubular bridge-piece which enables the Letus Extreme and the camera lens to be coupled together.