Carl, the comment about the 30's kind of answered the question, didn't it? It was probably a black steel buttweld pipe which had galvanize applied. There are plenty of things, including water, which will corrode or erode the galvanize layer over 60 or 70 years.
We get pinholes on the galvanized drain line coming from our garbage disposer. I don't know or care why that leaks but 50 years and the leak on the bottom of the pipe only may have something to do with it. It's tough to tell, but I don't think that the weld is at the bottom. I slapped some filled epoxy over it a few years ago and will not worry about it until it leaks again. PVC is a much better choice now. You might want to be sure that the thread areas are epoxy coated.