It's a screw up. I wish I'd known because I might have edited it out, certainly Dad's editor should have caught it ... but didn't. He had them a bit scared to touch his stuff because they had messed up a few times (doing what I would deem silly, as opposed to necessary, edits) and they were a bit gun shy to try to do too much.
These things happen. Just today I was working on a chapter that takes place in Venezuela in the 1960s and realized that in an earlier chapter I had called the Venezuelan Air Force the "Bolivarian Air Force" ... a name created by the Chavez regime because it sort of presets some sort of extra-national jurisdiction over (or with) other the nations liberated by Simon Bolivar in the 19th century. In the '60s it was just the Venezuelan Air Force. I had to zip back 10 chapters and fix it. Working without a word processor, and rarely researching in the middle of writing, Dad missed a few things. But that's what an editor is for.