I have to say that I don't understand your approach to my enquiry. Why are you not providing me with a helpful explanation of what is happening here, rather than just pointing me to the source code and say "go figure it out yourself". I have to question the point of having a LibRaw support forum here if the people who know the code won't explain why something isn't working in the way that someone trying to use it expected.

I was already aware of the annotated DCRaw web-site and have been there a number of times. On this occasion however I am still completely in the dark as to quite where in the LibRaw code the pre_mul value is set from the White Balance Coefficients, and very specifically I don't understand quite why pre_mul was set to values of 1.0 for the unsupported X-T3, but was set to useful values for the X-T1.

I don't want to add support for the camera in question to libraw in the current release. I realise that it will very likely be supported in the 0.20.? release as I believe it is already supported in the snapshots.