The D50-A is the calibration pin for the D50-SMA polishing disc for SMA style connectors. One in included with each disc, but can also be purchased separately here. To use this calibration pin, you will need an arbor press and a flat surface as a reference. Using the arbor press, press the calibration pin into the center hole of the puck. This will press the threaded part of the polishing puck into the flat disc until the end of the calibration pin is level with the bottom surface of the puck. This will ensure that the fiber is polished to the correct height in the SMA ferrule.