  We develop a numerical formalism for calculating the distribution with energy
of the (internal) pairs formed in a relativistic source from unscattered
MeV--TeV photons.
  For GRB afterglows, this formalism is more suitable if the relativistic
reverse-shock that energizes the ejecta is the source of the GeV photons.
  The number of pairs formed is set by the source GeV output (calculated from
the Fermi-LAT fluence), the unknown source Lorentz factor, and the unmeasured
peak energy of the LAT spectral component.
  We show synchrotron and inverse-Compton light-curves expected from pairs
formed in the shocked medium and identify some criteria for testing a pair
origin of GRB optical counterparts.
  Pairs formed in bright LAT afterglows with a Lorentz factor in the few
hundreds may produce bright optical counterparts (R < 10) lasting for up to one
hundred seconds.
  The number of internal pairs formed from unscattered seed photons decreases
very strongly with the source Lorentz factor, thus bright GRB optical
counterparts cannot arise from internal pairs if the afterglow Lorentz factor
is above several hundreds.
