The signals between childhood sweethearts in L. M. Montgomery's novels (plus a little bonus!):
"There was no telephone at the Gordon place and she and Jingle had agreed that when she wanted him specially she was to set a light in the garret window."
Pat of Silver Bush by L. M. Montgomery
"We've been...friends...ever since we were kids. Why, I used to put a light up in that little window in the attic whenever I wanted him over particularly and he'd sail across at once."
Anne of Windy Poplars by L. M. Montgomery
"Emily always knew when Teddy was coming, for when he reached the old orchard he whistled his “call”—the one he used just for her—a funny, dear little call, like three clear bird notes, the first just medium pitch, the second higher, the third dropping away into lowness and sweetness long-drawn-out—like the echoes in the Bugle Song that went clearer and further in their dying.
That call always had an odd effect on Emily; it seemed to her that it fairly drew the heart out of her body—and she had to follow it. She thought Teddy could have whistled her clear across the world with those three magic notes. "
Emily of New Moon by L. M. Montgomery
A bonus:
"She was not always a Princess, and he was not always a Pauper,—and that's where the story came in, I suppose," sighed the man. "She was just a girl, once, and he was a boy; and they played together and—liked each other. The girl used to signal, sometimes, from one of the tower windows. One wave of the handkerchief meant, 'I'm coming, over'; two waves, with a little pause between, meant, 'You are to come over here.' So the boy used to wait always, after that first wave to see if another followed; so that he might know whether he were to be host or guest that day."
Just David by E. H. Porter