Definition of "zipper"

I also competed in track and field at Idaho, and it's all because of putting the shot that I've got my long scar, my zipper, down the back of my neck.

1969, Jerry Kramer, Farewell to football, page 111

Making the transition from high school to college teams, for instance, he could not believe the scars his new players were sporting. "So many athletes had zippers down the side of their knee, or knees; they thought nothing of it and called this or that a Band-Aid operation. […]

1979, Don Atyeo, Blood & guts, violence in sports, page 227

[…] the usual gap between the time the last strike of the day departed and the first night hecklers arrived was closed with "zippers" — night fighters who, using day fighter tactics, proceeded to the target in daylight in time to relieve the last day blanket patrol.

1949, Walter Karig, Battle Report

They would handle all the night combat air patrols, the night hecklers who would go over the enemy airfields, and we formed a new little gimmick called "zippers" to help close that gap between the time the daylight strikes left the target and darkness.

1993, E. T. Wooldridge, Carrier Warfare in the Pacific: An Oral History Collection, page 268