Regarding vintage stainless steel spoke breakage:
I was told by a mechanical engineer at my work that the formulation of stainless steel (percentage of chromium) is critical to ensure adequate tensile strength without embrittlement.
This is illustrated by the problems I recently had with two Wipperman "stainless" steel chains. The first one exhibited cracking on 10 side plates, starting at the pins. This happened after only three months of riding. My Mechanical Engineering friend suggested that there was possibly too much chromium in the stainless formulation.
The second "stainless" Wipperman chain started RUSTING after only three rides. Apparently, there was not enough chromium in this chain.
Lastly (vintage content!), I was told years ago that Reynolds 753 obtained its improved tensile strength because additional carbon was added to the formulation, in addition to the heat treating process. The additional carbon made the tubes stronger, but also more brittle. I recall reading the percentage of elongation (stretching before breakage) for 753 was distinctly lower than 531 or Columbus SL.
Andrew Gillis (riding conventional carbon steel chains now...) Long Beach, CA