I have a lot of regard for most 'Gun Book' Authors. It is a lot of hard work and investment of time, done for the love of subject. It can not be very financially profitable.

Putting research, observations and conclusions into print, opens one up to criticism, disagreement, second guessing, and fault finding.

New information has a way of coming to light afterward. This can invalidate assumptions and make insights and beliefs look foolish, naive, and even stupid.
(Some critics seem cruel and hateful. This seems to even happen on other gun forums).

I have a tremendous regard for the works of William Brophy. He shared a wealth of knowledge through his books. (It is probably useful and helpful for us to respectfully 'flag' minor errors).

IMHO - All gun books will have some errors. (As Whig stated, the 'headless cocking-piece is trivia). We get a broader view and can advance the subject, when we discover errors and exceptions.

I imagine Joe Farmer's book may throw more light on the 'headless cocking-piece' story, if you have further interest.