Is the 661-7014 screen backwards compatible with 661-6594?

You can’t mix unibody and retina screens!

Just like how TV changed how digital TV over the air signals can’t be accepted by an analog TV without an adapter. Video signaling also went over a sizable change as well from the older LVDS to eDP or iDP. So while the physical displays connector inside stayed the same the signaling and power lines where altered! Between the old and new signaling setups.

Also note even the video signal with a slight change so the first Gen retina displays use eDP and the last two used iDP when the signals again changed! So don’t assume a newest generation of Retina displays will work either.