I am in the process of trying to determine the parentage of Charles Floyd of the Lewis & Clark expedition. I lean toward N.J. Floyd's account, as N.J.'s father would have been Charles's brother. In fact, N.J.'s father wrote a letter in 1804 to his sister Nancy that mentions the death of "dear Charles."
I am not familiar with the 1966 work you refer to, but I do know that George H. Yater & Carolyn S. Denton in "Nine Young Men From Kentucky" (WPO Publication No. 11, May 1992, 24 pages) include a biographical sketch of Charles that reports his father as Robert Clark Floyd. They may have used your 1966 source. I am in the process of buying the publication and will let you know their source. It is the Yater publication that is frequently used in alot of Lewis & Clark literature - I believe incorrectly.
The Yater publication also reports that land granted to Charles posthumously was deeded to his brother Davis, and sisters Mary Lee and Elisabeth. Again, these individuals are reported by N.J. Floyd as children of Charles, Sr. and not Robert. What does your 1966 source say about Davis, Mary Lee and Elisabeth?