Hi marc,
I think you are referring to a lead "closet bend", that was usually installed when caste iron was used for most of the waste & vent system. Sometimes lead drains from sinks/tubs/showers were also present, but often already replaced. These lead bends were used mostly from the very end of the 19th century through the 1930s. Most common during the teens and '20s. This is the time that many cities passed ordinances prohibiting use of lead for more than 2 feet. I've encountered some from the early '40s, but these are almost always in government or military buildings.
Here, during the 1930s, K&T phased out to conduit and BX before non-metallic cable was used. This is in the towns and cities. Rural areas often didn't have electric until NM was pretty standard. Of course, a majority of the rural buildings I inspect had plumbing & electric added long after initial build.
I think structural components, millwork and hardware more reliably reveal more about a building's age than systems.