We analyze various calculated fission-rate ratios in the leakage spectrum of the NIST iron sphere benchmark field employing a few modern cross-section libraries and compare the only measured value of the ²³⁷Np/²³⁸U ratio to the respective calculated values. We also present the sensitivities of the calculated fission rates and fission-rate ratios to the ⁵⁶Fe elastic- and first-level-inelastic-scattering cross sections. We conclude with a recommendation of additional measurements that can shed more light on the various iron, in particular ⁵⁶Fe, cross-section evaluations.