  A parameter-free and relativistic extension of the RuhrPot meson-baryon model
is used to define the dominant isoscalar meson-exchange currents. We compute
pp-bremsstrahlung observables below the $\pi-$production threshold using a
relativistic hadronic current density that includes impulse, wave function
re-orthonormalization, meson-recoil, $\bar{{\rm N}}$N creation and
annihilation, $\rho\pi\gamma$ + $\omega\pi\gamma$ + $\rho\eta\gamma$ +
$\omega\eta\gamma$ vector-meson decay and N$\Delta\gamma(\pi,\rho)$ exchange
currents. We obtain a good description of the available data. The
N$\Delta\gamma(\pi)$ current is shown to dominate the large two-body
contributions and closed-form expressions for various non-relativistic
approximations are analyzed. An experimental sensitivity to the admixture of
pseudo-scalar and pseudo-vector admixture of the NN$\pi$ interaction is
demonstrated. We examine the Lorentz invariance of the NN$\rightleftharpoons$NN
$t$-matrices and show a dominantly pseudo-vector NN$\pi$ coupling renders
impulse approximation calculations without boost operators to be essentially
exact. Conversely, a similar analysis of the $\Delta{\rm
N}\rightleftharpoons$NN transitions shows that boost operators and the two-body
$N\Delta\gamma$ wave function re-orthonormalization meson-recoil currents are
required in NN, $\Delta$N and $\Delta\Delta$ coupled channel $t$-matrix
applications. The need for additional data is stressed.
