This BETA mount was designed to modify and adapt a nFD or FDn 100mm f/2.8 lens (34mm rear element diameter) to use it with EF DSLR Cameras, along with this part:
https://shpws.me/SlYP arm for nFD 85mm f/1.8
For more aperture arms please visit the store main page:
https://www.shapeways.com/shops/lensmounts
Follow this link to see the tutorial:
https://youtu.be/Jiv1HAlwF68