With older projects, even when all of the documentation and the blueprints are still there, it is often the case that the expertise required to read and understand them is no longer there since industry standards for documentation change over time, those who enter the field today learn only the modern standards and those who still got to work with the old standards have retired. I'm not saying that this is definitely the case here but i would not be surprised if the younger generations of engineers who are used to CAD methods would have some struggle understanding old school technical drawings.