The Mabie Todd Blackbird Topfiller was the lower trim version of the Swan Visofil. The Blackbird Topfiller fountain pen is a linear compression filler with the barrel as the ink reservoir. It has a lot in common with the better known Parker Vacumatic. Remove the blind cap to get access to the filling mechanism. The mechanism is manually compressed to push air out of the barrel via the breather tube, and ink is drawn in. Repeat about 10 times to fill.
This pen is similar to the Swan Visofil VT, except it lacks the elegance. Here you remove a blind cap and manually compress the sac-over-spring, as opposed to the concealed mechanism of the Visofil VT with the extending button.