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.
Researcher, far as I know, 24's have never been chambered for anything other than 2 1/2". Or in any case, all the shells currently available are 2 1/2". Nice oddity sort of thing, I guess, but you end up with a gun that has a slightly larger bore than a 28, but shoots a slightly lighter standard load (11/16 oz vs 3/4). And you can always reload for the 28, fairly easily. I seem to recall that the late Don Zutz had a 24ga reloading setup from Mec.
John, I had a Beretta 24ga. Are you talking sxs or OU on the ones you imported? I don't remember your name on the one I had, which was an OU with DT and sling swivels. I actually liked the gun, picked it up cheaply enough (at Cabela's), and figured I'd use it on occasion for grouse and woodcock. But the problem was, it had pretty tight chokes and chrome bores. Couldn't figure out how to get them opened, so it moved on down the line.
