Like other variations of sleeve filler vintage fountain pens (barrel extension, rotating cover, rotating section, end knob activated), there is a covered opening in the barrel so the user can directly press on the pressure bar and sac for filling the fountain pen after moving the cover out of the way. Two versions are presented that each use an external sleeve that slides on the barrel to reveal the pressure bar and sac.
The Waterman sleeve filler (pictured first) is the better known version of this style. The sleeve simply slides toward the end of the pen. Internally there is a mechanism to prevent simply sliding the sleeve off of the pen. This pen was produced from around 1910 – 1915. I believe that the patent was US 950817 issued in 1910.
The John Holland Eureka (pictured second), described in US patent 821940 issued in 1906, used a long slit with a short right angle turn at the end which rides on a pin to limit sleeve motion. It also has two long cuts, which I can only assume is to facilitate expansion and sliding onto the pen. The sleeve is slid up towards the nib to expose the pressure bar.