My latest theory is that one of the MASTER files has an object that’s linking the the “Steel_bolts” material from the chassis master file that’s in the wrong directory. Unfortunately since most MASTER files are using the chassis (due to it being the backbone of the whole DOG) I’m afraid the only way to fix this error is to remove the stray material.
I say “unfortunate” because each file would need to be checked 😵 Blender doesn’t make the tracing of an “indirect library link” very easy. For rendering I will ultimately delete all materials from the master files anyway, applying my own. I suppose at the very worst that’s when the problem will be solved.