Okay I have now gone through two supplies to triple-check I didn't miss something. One with the fault above and one working and swapped every soldered-in riser board in an attempt to isolate which board(s) it could be.
-Swapped the main heatsink and it's components (bridge rectifier, PFC and MOSFET's) - no change
-Swapped the PWM/PFC control board - no change
-Swapped the secondary rectifier heatsink assemblies (both of them) - no change
-Swapped the DC-DC regulation module - no change
-Swapped the output distribution board - no change
The fault is on the main PCB. On a good PSU the two bulk primary DC filter capacitors charge up to a little over 400v but on the bad board they will peak at 260v DC. Power to the PFC/PWM board is also unstable. The PFC feedback winding on the main choke is not open. It's something early on in the DC boost but I cannot find a bad diode or resistor both visually or with the DMM.