We know before Microsoft decided to retrench Windows 10 Mobile the Lumia 950 was meant to be a much more interesting handset, including support for the Surface Pen.
Now an early prototype of the handset has leaked, and website Proto beta Test have posted a video showing the Surface Pen in action.
Like recent Surface devices, the handset uses N-Trig technology, making it compatible with the current Surface Pen.
The handset as originally envisioned would have featured active covers and smart sides, making for a much more interesting device than the boring handset eventually delivered.
Microsoft is rumoured to be working on a folding Surface Phone which would almost certainly offer Surface Pen support but the device is not expected to be widely distributed. Instead, if you want a smartphone with a pen now Microsoft is currently selling the Samsung Galaxy Note 8, considered by many to be the best smartphone on sale currently.