  In a previous article [arXiv:1011.4258], we established the consistency of
isotropic modified Maxwell theory for a finite range of the Lorentz-violating
parameter \tilde{kappa}_{tr}, which includes both positive and negative values
of \tilde{kappa}_{tr}. As an aside, we mentioned the existence of a physical
model which, for low-energy photons, gives rise to isotropic modified Maxwell
theory with a positive parameter \tilde{kappa}_{tr} (corresponding to a "slow"
photon). Here, we present a related model which gives rise to isotropic
modified Maxwell theory with a negative parameter \tilde{kappa}_{tr}
(corresponding to a "fast" photon). Both models have an identical particle
content, photon and Dirac particles, but differ in the type of spacetime
manifold considered.
