This is a bulb filler.
Since the barrel is a translucent material, you will see the ink through the barrel, and know when to refill...
Here's a video of a bulb filler being filled. Not this pen, but you get the idea as to how it fills.
[yt]<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/SFUUx5bjd3E&hl=en&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/SFUUx5bjd3E&hl=en&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>[/yt]
Since the barrel is a translucent material, you will see the ink through the barrel, and know when to refill...
Here's a video of a bulb filler being filled. Not this pen, but you get the idea as to how it fills.
[yt]<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/SFUUx5bjd3E&hl=en&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/SFUUx5bjd3E&hl=en&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>[/yt]