During World War II, the government ran a program called the Army Air Forces Ground Observer Corps (GOC), which placed over 1,500,000 volunteers, largely women and teenagers, in observation posts across the country. Their mission was to scan the skies for aircraft and compare their silhouettes to a guide of enemy aircraft. Although not a part of the GOC, the Navy Yard program seems to have operated similarly, as evidenced by this description in the Navy Yard Shipworker, the company newsletter.
Thanks to Navy Yard archivist Elizabeth Mc Gorty, I was able to get a copy of the original drafting of these structures, dated 1942.
I've found two of these structures in the yard. I don't know if there are any others around, or how many there originally were, or exactly where they were located. They have moved around three times in the time I've been visiting the Yard, as though they don't want to junk these structures, but don't know exactly what to do with them.