Filmstrip sprocket holes appear on the left side of the image in the Viewer to indicate the first available frame of media, and appear on the right side to indicate the last frame.
An L in the Lower left means you are on the first frame of the edited clip, and a reverse L in the lower right means you are on the last frame.
Press Option - S
Click the Snapping button in the Timeline.
Click the Snapping button in the Viewer.
Click in the empty gray space above or below the track.
Press Shift - D
Press Option - D
Press Command - Shift - A