Thanks! After closer inspection with the grease out, it seems to works in this manner:
1. It appears to work off a suction process to advance the piston.
2. The end of the piston is smooth, and there is no threading.
3. The double winged "keyhole" opening is probably meant to initially advance the piston after refilling. This double wing is to allow for a twisting action of the rode to keep the plunger from cocking to one side. When you use a straight rod to push the plunger, it tends to cock to one side and bind.
This little grease gun is a nice addition to my lube tools.