These are the lengths of wood on the reverse of the wooden surround which attach it to the back box.
Stretchers/Rails are all made from period mahogany/oak or pine.
Dependant on age of clock or just copying an original one, there are many different styles and sizes to choose from, this will be discussed prior to the making of them. The original backbox would be required to ensure the peg holes line up with the existing holes in the case.