“Using this cable I've successfully connected a 27" iMac late 2009 with iMac Retina 4K, 21.5-inch, Late 2015 using target display mode.
After visiting the Apple store numerous times, without them being able to offer any solution or help, I ordered this cable on a whim with the warning from Macfixit that it may not work. However, wanting to make use of my old iMac I was keen to give it a try. Easy to set up and works like a treat, thanks Macfixit!”