  Here we point out that an observation of Ultra-High Energy Cosmic Ray (UHECR)
photons, "GZK photons", could provide an upper limit on the level of the
Extra-Galactic Radio Background, depending on the level of UHECR proton
primaries (to be determined after a few years of data taking by the Pierre
Auger Observatory upgrade AugerPrime). We also update our 2005 prediction of
the range of GZK photon fluxes expected from proton primaries.
