Bront; Huon Pine was chosen as its a naturally oily timber, highly resistant to moisture decay. Its traditionally a rare practice to stain the inside of an enclosed box. Had the outer box remained a light coloured timber, its surface would have been susceptible to later unsightly staining from the combination of lubricant and stone slurry. The use of a caulking seal to prevent moisture entering the lower housing of the box is self explanatory. Traditionally, tar/pitch was used. As a reference to historic practice you may find the following informative.
https://www.google.com.au/search?q=i...il+stone+boxes
Appreciate your questions.
StewieS;