Heat exchanger body is cast iron, hence possibility of rust stains ... Your water lift muffler
(?) is probably glass, but older steel
bodies can rust as well. Heat exchanger tube stack is usually copper bronze. There should be a "pencil zinc" mounted to the 1/4" drain plug
on the exchanger. (Camp E-0 3/8" x 1-3/4") to reduce corrosion
of the cast iron body. Many times the zinc portion of the plug
assembly has corroded away completely and therefore not replaced. Would be my first check as an exchanger body is an expensive replacement.
Hope this helps