Hello Bill,
Yes, these oil coolers are steel and are brass welded together. They must handle very high pressures when engine oil is cold. Typical radiator construction of soldered brass and copper would fail.
They can rust from the outside due to road salts, age and weather. When they are nearly impossible to repair with confidence. New ones are very expensive. The W108 six cylinder sedans of the era (six cylinder carbureted or injected) uses the same cooler. You are lucky to find a good solid used one!