Until 1947, the earliest Hebrew manuscripts available to serve as the basis for Old Testament study and translation were the Massoretic Texts of eastern European Jews. These texts of the Hebrew Scriptures date from around 900 AD. The translators of the Authorized Version (King James Version) used these texts as the basis for their Old Testament translation.
Besides the Massoretic texts, the Christian Church had always used the Septuagint, which is a Greek translation of the Hebrew and Aramaic Old Testament made around 200 BC. The Septuagint was used to compare with Hebrew Massoretic Texts to check meaning and accuracy. In the 1800 and 1900s other early Old Testament documents were discovered, adding more textual information.
Add to all of this, the discovery of the Dead Sea Scrolls (Qumran) in 1947, and the huge impact made on biblical scholarship by these very ancient documents. The Hebrew and Aramaic scrolls of Qumran date from the 200s BC to the 100s AD and include every book of the Old Testament except Esther, as well as other kinds of writings. These have been compared with the Massoretic Texts, the Septuagint and other manuscripts to discover how much the text of the Hebrew scriptures may have changed over time as manuscripts were copied.
The result was the amazing fact that little or no significant variation occurred in more than 1,000 years of copying from 200 BC to 900 AD. The only major differences in the texts were the Massoretic invention of Hebrew vowel points as a refinement over the mainly consonantal biblical Hebrew.
So, the tradition that Jewish scribes used extreme care in copying the scriptures proved to be correct and those who study and live by the Old Testament can do so with confidence.