The Rider pen is another version of a “jointless” fountain pen like the Hicks Sackett and the Parker Jointless. To avoid the potential for ink to leak on your fingers at the section joint, there is simply no joint at what is normally the section end. The barrel and section are a single piece. To fill this variation of a jointless fountain pen, you remove the feed. The feed has a notch on the underside. The clip of the pen is designed to engage the notch, and the feed is pulled out, leaving the nib in place. The pen is then dropper filled though the opening where the feed was. Then simply replace the feed. The feed has a triangular shape that ensures it can only be inserted in the correct orientation and depth when it is replaced.
Thank you to Otto Yang for photos and Todd Eberspacher for the patent.