What They Saw
The more pure the nitrogenase, the less protection from oxygen inactivation they observed. But while crude extract had the most protection, more pure forms were pretty similar until the protective FeSII protein was absent, in which case the nitrogenase was rapidly inactivated. Magnesium ions (or possibly other divalent ions) were also necessary for this stabilization.
This protective protein was 14 kDa, orange in color, and had 2 Fe and 2 S atoms, so a 2Fe-2S center (thus the name). This version seems smaller than the A. vinelandii version though, which is 23 kDa. In stable complexes, the three components (dinitrogenase, dinitrogenase reductase, and FeSII) were present in about 1:1:1 ratios.
What This Means
This FeSII (with Mg ions) appears to be sufficient to protect the nitrogenase complex from oxygen, stabilizing it even outside of the cellular environment. This stability is perhaps not complete though, since crude extracts did show more activity after exposure to oxygen.