This piece controls the aperture arm, goes between the black aperture ring and the two posts. It was designed for short focus barrel movement. Tested on the nFD 50mm f/1.4, nFD 24mm f/1.4, 17mm f/4.0 and nFD 24mm f/2.0
Follow the link to see the tutorial:
https://youtu.be/Jiv1HAlwF68
Please visit the store for more aperture arms:
https://www.shapeways.com/shops/lensmounts