Back in the first forty years of the twentieth century, when American loading companies were regularly providing 24-gauge shells, the only length I find offered was 2 1/2 inches. In those days the standard load in the 2 1/2 inch 24-gauge shell was 2 drams equiv. and 3/4 ounce of shot, while the standard load in the 2 1/2 inch 28-gauge was 1 3/4 drams equiv. and 5/8 ounce of shot.