The right-hand tail of the theoretical blackbody spectrum I showed before follows the Raleigh-Jeans law. It looks like it falls off faster in your Figure 3.4 because wavelengths and intensities are shown on log scales there, but it is the same curve as I show. Rossi's SK spectrum falls off much faster.
Agreed that the log scales disturbes the picture and that Rossi's spectrum falls off much faster and I have no clue why this is the case.
However I don't know if the vertical scale of Rossi's spectrum is representative for the energy.
Maybe Paradigmnoia can say more about this since he seems to know the type of equipment used for measuring the spectrum.