The Eagle Snap Shot is thumb filler fountain pen mechanism with an internal cover operated by a barrel end rotating knob. When the pen is in use, there is a metal cover blocking the hole in the barrel. Twist the end knob to expose the filling bar and fill the pen.
The Eagle Snap Shot was more typically seen in black enameled metal with a slip cap and brass (or plated?) nib also labeled Snap Shot. The higher end ebonite model has a 14K Eagle Shield logo nib.