@johonline as @deckingman said, check what version of Duex5 you are using, and RRF version.
"is the firmware clever enough to detect the board version as well as the board type"?
Yes, but only if you are using RRF 3.4 and later. See https://docs.duet3d.com/Duet3D_hardware/Duet_2_family/DueX2_and_DueX5#pcb-revision-history
Modified the series resistors of the temperature inputs to be 2k2 so they perform better with PT1000 sensors. From RRF3.4 the DueX5 version will be detected and the correct R values set in M308, for RRF 3.3 and earlier use R2200 in the M308 command.
Ian