Thats a beauty Dave! Why is there a hole in one of the corks though?
That is for the articulated C#/G# mechanism. Normally, the C# key is located on the upper joint, but the pad is kind of squished right to the very end edge of the joint, leading to some intonation issues.
The so-called "articulated" mechanism seeks to solve these issues by placing the C# hole where is
should be, which happens to be exactly in the center of the middle tenon. It's kind of a complicated thing where they have to drill a hole through that tenon but it is said to make the C# a better sounding note.
This problem arises directly from the breaking up of clarinets into two main body joints instead of one (unibody) setup. Unibody clarinets naturally don't have this issue because they hole can be placed exactly where it needs to be, without any hindering of body joint breaks.