That's probably what the Author, Doug Stewart, is referring to near the beginning of this interview by Nick Larson on the Project Upland podcast. Stewart kind of sheepishly says to ignore the grammatical errors in the book. That he really does know a lot about SxS's and he is aware of how the "grammatical errors" might make it seem like he doesn't. He repeats the apology and they talk about the typo at length, without specifically saying what the typo is, at around 1:01:00. The interview is from Aug. 23/18.
https://www.dougstewartauthor.com/news/
I didn't realize that Doug Stewart was a PGCA member. I see he had a post a while back to introduce the book. Had I known that I would have let him respond to the spelling question.
Here is a link to the post in case you missed it like I did.
http://parkerguns.org/forums/showthread.php?t=24723
Threaded Mode