I think it is p/n HN-1 Screw in Product # 508 NCP. \
Also fits PC 35/2.8, AF 24/2.8D, 20-60 1x nikor.
I got this in my Nikon product Guide 1999.
I would call Nikon Parts.
My son needed to replace a part in an underwater housing.
He was calling it seems everybody in Florida to no avail.
I said call the manufacturer- well he finally did and they shipped it via air that afternoon.
Birger