I have a Leaf Aptus II 10 listed on Luminous with a Mamiya mount at a very good price, but don't know if this would work on your HY6. It's been a great back and wonderful to make large prints from.
I also have a whole Mamiya AFD II system I will be listing with the following lens: 45mm AFD, 50mm shift (manual), 55mm AFD, 80mm AFD, 120mm AFD, 210mm AFD.
All equipment is in very good shape and could be packaged with my Aptus II 10.
Feel free to contact me via private message if you have questions.
Drew Harty