This model is a replacement button for a sleep apnea machine that connects over a circular rod with a flat notch. The machine is discontinued, as are replacement parts (plus, isn't it better to fix it yourself when you have the tools? :D )

The button top as printed turned out a bit stringy, (looks like I've got some printer settings to adjust), but in the model it has a slight bevel, with the aim of reproducing the tactility of the original (now lost) button. 

Rod mechanism and button models are included. Fit was very tight. If I were to re-print, I would increase the inner diameter by another 1mm (might be fine on a more “dialed in” printer).