29E31S and PFX KW0606A yokes are compatible. Very close readings. I just used an E31S yoke to test a PFX chassis and it worked fine for the 10 minutes or so I had it powered on.
Also, I just finished setup on a donor flat tube (LG A68QCU770X) using a curved 29E31S yoke and a chassis labeled "29PF31", which I assume is the PF version of the chassis. The yokes between flat and curved are indeed different, and you will need to compensate for the bowing by adding more magnets around the perimeter of the yoke and around the edge of the tube to get a decent result. It also has some static convergence issues on the top and bottom which I couldn't solve trying any of my spare ring sets or anything. I am currently looking for more PF or PFX yokes to test with.