"The elbow appears to be cast iron."
You must be sure, not only believe that it's cast iron. With a file, file a few decigrams of material from a place that won't cause any harm and take it to a chemical lab to run a chemical analysis.
I personally would say that it's not cast iron. It's a long time that cast iron fittings aren't use for steam any more.
Giovanni S. Crisi
Sao Paulo - Brazil