I sold all my older Aquatica 3 housings for Nikon F-3s long ago and just found this part for that system. Its custom made 3/4 inch anodized aluminum extension ring . It is about 6 inch diameter threaded with o ring. Great condition.Been boxed for 2.5 decades.
These fit all the older Aquatica housing #'s and 4's before bayonet mounts came along.I was dealer back in the day and had many custom rings made for various lens.
Its free for the shipping cost if you can use one. Otherwise its scrap soon.