Made this last year. It's an arbor for mounting the small Sherline chuck on my big lathe, since the Sherline chuck is much more accurate for small work. There's nothing particularly fancy about it - a standard MT3 blank-end arbor is held in the spindle with a 3/8"BSW drawbar. The end is drilled and threaded for a small aluminium adapter, which in turn holds a Sherline chuck-to-rotary-table adapter (the black steel part with the threaded ends). I did it this way because I didn't fancy turning a ¾" thread directly on the end of the arbor; besides, the arbor diameter wasn't enough to leave a flat for the back of the chuck to locate against. I marked the location of the arbor in the spindle taper so it can be replaced in the same place each time, just in case there's any slight variation.
The scroll on the chuck does have a slight tendency to loosen sometimes when starting the lathe due to the initial kick - some tape around the edge of the chuck helps prevent this.