As the first picture shows, the tested surface is a small and long hole, but the driver is too big to go inside. In this case, we need the help of extension rods. The hole is 170mm long and we use three extension rods to reach the farthest part inside the hole.
From the last two pictures, we can see that the accuracy is still guaranteed when we use the extension rods. The test value is 1.645μm for standard block 1.64μm.