So maybe we can deduce that by '72 Columbo was labeling the tubesets (that is, including the foil stickers with the package) and so probably using the SL designation.
Maybe somebody can further refine the History...what Columbo/Columbus has published themselves kind of glosses over the period from 1930 to the early '70s, but we know they were cold-drawing seamless reinforced (double-butted) chrome-moly tubing for bicycles in those years, and it was being built into (mainly Italian) frames.