Esterbrook nib adapter. Fits to their Estie pens, allowing them to use a vintage Esterbrook nib in place of the modern nibs supplied. Why would you want to? Well, there are a lot of vintage Esterbrook nibs out there, at low prices, and in lots of different types, so it's a great way of being able to use a more suitable nib if you prefer something more unusual, or to try out some of these vintage nibs without using a vintage pen.

Not that there's anything wrong with using a vintage pen - plenty of people do, and love them, and many of them can be great pens. They can be a bit more finicky and a riskier purchase than a modern pen, though, so for some people, this might be the perfect compromise.