Mike,
I found my original data sheet for my CDI furler (attached). It still doesn't show that part. But you can see the two sheaves on the drum and the cleat in the picture. That pin on the part you have goes through the tack of your jib. Then the line goes down to the first sheave on the drum, back up through the first sheave on the part, and so on. You get a 4 to 1 mechanical advantage to tighten the luff, then cleat it. The entire thing just spins around.
Depending on which model you have, you may have the foil with the internal wire halyard or like mine, a locking mechanism at the top. It requires that unlocking tool to raise and lower the sail from a locked position.
I did replace the original plastic drum with a CDI metal drum with ball bearings. It has worked fine for us for 34 years.
Allan