I have NO interest in reviving a vendetta, but am simply trying to give the rest of the story regarding this black stuff
Rib extension wedge left - braze and contamination center - barrel steel right
The composition by EDX of the areas are at the bottom here
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=546106&page=10 re: "dirty steel". The metallurgical (non deep thinkin') version would be steel contaminated by metal oxides, silicate, sulfide, and manganese sulfide inclusions.
"Scale" is on the surface of forged steel and is black iron oxide, and obviously could contaminate a braze or weld joint.
The comment by the metallurgist last week:
The braze contamination descriptions are kept vague because that's about as conclusive a statement we can make based on the data. The concentrations of iron and oxygen detected could technically be consistent with corrosion (rust) or overheated steel. Based on where it was observed in the braze, we need to refer to it as ferrous braze contamination. We don't know exactly what it is. This could have occurred during the brazing process (likely) and been exaggerated over time by successive heating cycles, moisture, etc via possible alloy segregation effects, electromigration and such phenomena.
By the metallurgical engineer today:
What I mean by dirty braze is that they may have not controlled the environment well or cleaned properly. Also, the braze material itself may have had contamination in it. We can not say for certain but since it is such an old braze.
This doesn't really add much to what several here postulated from the start, but might be of interest. Thoughtful comments/corrections are of course welcome.