Is the decorative end piece. Its only function is to hide the joint where the two sides meet, a difficult joint to make meet perfectly, so most luthiers cut out the joint and insert a pretty piece of wood. so do I. It starts by routing out a gaping hole and cutting away the black/white/black purfling.
Next, you glue in some black/white/black down the sides of the gap with super glue. The wet stain is the sprayed on glue catalyzer for a quick set. It is not permanent.
Now the challenge is to fit the end piece into the slot. That is done by sanding to fit.
 I you cut the end piece the correct angle then as you sand the sides it moves further and further up the slot, |  'After a little sanding it is progressing up the slot |  Another bit of sanding brings it closer, needs just a bit more |
 Back to the sanding block for a wee bit more |  AHA ! a perfect fit. Now apply the glue and tap the endpiece in . |
A bit of tape will hold it tightly as the glue dries and tomorrow I will sand and scrape that down to its finished condition. I will show you the result on the next blog.