The quoting got too confusing so I'll just add some clarification and thoughts:

The thickness of the clamp was .065" 304 stainless and the extension was .125" 304 stainless. The extension didn't crack at all so I suspect the difference in thickness forced the stress/contraction into the clamp material in addition to requiring a little more heat than 2 pieces of .065 would.

Stainless doesn't transfer heat like 4130 or other standard steels so when it comes up to temp, the heat stays very localized and goes from too cold to perfect to 'dang, now I have to start over' in the blink of an eye. After this experience, I'm thinking a bigger/softer flame might help spread the heat and not keep it so concentrated as typical O/A fillet brazing flame does.

NS is a PITA to file! I don't even try. Frankly, I don't like tinning with it like Paul does. Mostly out of fear that something is going on between the layers that I don't understand.

JohnDThompson I use NS fairly regularly with Gasflux B on steel with good results. My sense is using NS, B flux and stainless is just a combination that is fraught with potential problems.