No.
The letter at the end of the build number stands for the number of revisions (or compiles) for that specific build-number. First build of let’s say „19B5068“ in this case would be 19B5068a. If a major compilation-error is found, another compile is done, named 19B5068b, the third compile would be 19B5068c and so on.
In later stages of beta versions, the chances are higher, that a first (a) or second (b) compile of a specific build is considered good for release. That‘s why late betas often end with a or b. 😉