This is a Nikon 39mm filter holder. Nikon 39mm filter holders are part of a drop-in filter system used with specific lenses. They are primarily attached to telephoto and super-telephoto lenses, allowing filters to be inserted inside the lens rather than on the front. This allows for the use of relatively small filters even with large-diameter lenses. Compatibility is limited to certain Nikon lenses, so be sure to check compatibility with your lens before purchasing.