This was one of the earliest fountain pen models, and pretty simple to fill because the barrel of the pen held the ink. The section was unscrewed from the barrel of the pen and an eyedropper was used to fill the barrel with ink, up to about a quarter of the way up the barrel. Then, still holding the barrel upright so as not to spill the ink back out, the section was screwed back onto the barrel. Making sure it was tightly secured so as ink did not seep out (which it tended to do regardless--I have found mine has a fair amount of condensation (or even leakage) into the  slip on/push cap if left filled with ink (or even the user as the pen is being utilized), making this quite a messy, but nonetheless interesting pen to write with). Almost all earlier fountain pen companies started production with this pen.