Dobro's and Nationals have difficulty with neck joints being loose and often times leaving the playing action very high. This is not a problem for those who only play slide but it's a big problem for those that want to mix it up and fret the notes as well as play slide.
Adding the bolt as pictured will put an end to this problem and hopefully this series of photos will help in understanding the process for doing this. This is also a good time to make any adjustments to the neck angle if needed to achieve a specific action height. While I had the instrument apart it was a good time to take care of straightening the neck and refretting as well as taking care of a number of other structural issues.You may note that this needs to be done with great accuracy or you could end up doing more harm than good.