The first lever filling fountain pen is attributed to Sheaffer in 1907 (although I have read about an earlier lever by Whitney on another website).
Regardless of how the lever is attached to the barrel, the principle is the same. So for now, I will not be working too hard to show examples of lever attachments by C ring (everyone), barrel pin (Sheaffer), lever box (Waterman), etc. I will show my odd pen with external C ring because it is unusual.
I believe that the pictures tell it all. But for those who prefer prose, I will try to describe it. The lever is a length of metal (or hard rubber on some Swan pens) attached to the barrel roughly 3/4 or so down the length. Lifting the end of the lever causes the far end of the lever to pivot into the barrel, thereby pressing on an internal pressure bar, which in turn compresses the sac.