This is a 52mm adapter ring, for use with the Nikon AF-4 filter holder.
Urth 86-52mm Adapter Ring for 100mm Square Filter Holder
Nikon 72mm Adapter Ring for AF-4
Nikon 67mm Adapter Ring for AF-3
Nikon 62mm Adapter Ring for AF-4
Nikon 77mm Adapter Ring for AF-4
Nikon BR-2A 52mm Reversing Adapter Ring
