This is the white plastic moving block (about 3” X 4”) that the chair mounts to on a Betadrive system (if you have a blackblock, you will need our part #6253); as the block moves up and down the stationary motor shaft, the chair goes up and down.  There are primarily two white moving blocks used in the USA; the difference is in the thread size in the block where the worm screw goes through the center of the block.  The way to tell which you need is to look for the slots in the aluminum shaft (one on the top and one on the bottom of the shaft); shine a light into the slot and see the black worm screw.  Using a tape measure or ruler, pick out a tread on the worm screw, and count how many threads (or turns) there are to the next inch on the ruler… It will be either 6 threads (our part #6254) or 9 threads (our part #6256) per inch.  The 9 thread per inch size is by far the most common, but there are enough 6 thread blocks in use to make sure that you make the effort to get the right measurement.  It is very important that you check the thread size before you order a moving block, because if you order the wrong one and try to install it on your motor, we can’t take it back.  Installation instructions are listed on the Okin Troubleshooting Guide.