In 1969 Michael H. Kater wrote a study on the Earnest Bible Students in Vierteljahrshefte fur Zeitgeschichte 17: pp.181-218, in which he estimates 10,000 Witnesses were imprisoned and four to five thousand lost their lives. Christine King published this as fact in her book The Nazi State and the New Religions, New York, 1982. The Watchtower of October 1, 1984 had a report on King's book which included these figures.
When it refers to 10,000 who were imprisoned that includes all forms of imprisonment, not just concentration camps. So while absolute figures are difficult to determine, the figures in the 1974 Yearbook have been provided as verified. It is reasonable that the actual figures were much higher. In his book Between Resistance & Martyrdom, London, 2007, Detlef Garbe concludes it is likely accurate that 10,000 were imprisoned but that the number of deaths was closer to 1,200 than the four or five thousand estimated by Kater.
Garbe adds (p.483) "In summary, the numerical data in the Watch Tower Society's 1974 report about the Bible Students Association in Germany is a remarkably reliable source".