I had read on the internet, early 30s.

This is a catalog from 1935 with an aluminum stem, not Ambrosio, but no reason to believe Ambrosio didn't have one at the time. https://plus.google.com/photos/10861...34556256340805

My bike came with a steel stem, I gave it an "upgrade" of an aluminum Ambrosio stem and bars from 1939. A scratch on the aluminum makes it look like 1959, but the date stamp is 1939.