[en] Correlated measurements of red-shifted iron line emission and apparent surface brightness are suggested for unambiguously defining intrinsic x-ray characteristics for clusters of galaxies up to z greater than or equal to 1. If some of the weak, unidentified high galactic latitude x-ray sources are clusters at z approximately = 1 to 3, then such correlated measurements should be feasible witin the complement of instruments aboard the HEAO-B orbiting x-ray telescope observatory. In addition, those clusters at z less than 1 would require spectral data from broader bandwidth experiments, such as the all-sky survey to be provided by the proportional counters aboard the first mission of the High Energy Astronomy Observatory (HEAO-A)