Who had the depth of expertise in those specific areas to organize that expertise into a usable reference framework.

Mary Ross was one of those people.

That is what her co-authorship means, in specific technical terms: she had the expertise to contribute meaningfully to the handbook because she had worked through the underlying problems deeply enough to understand their structure.

That understanding was not acquired quickly.

It was the result of the years of mathematical training she had done before Lockheed, the computational work she had done in the early years at Lockheed, the advanced engineering study she had pursued during her time there, and the specific project work in missile and space systems that had given her direct experience with the problems the handbook addressed.

The handbook is the most publicly accessible documentation of a body of expertise that was much larger than the handbook itself.