A slow forward shock-type discontinuity observed by Helios 1 on day 147, 1980, at 0.31 AU is analyzed in detail, employing high-resolution (40.5 s) plasma and magnetic field measurements. It is shown within the bounds of uncertainties in the various parameters (1) that this event is not an MHD discontinuity, as the total pressure balance is violated and as the entropy increases across this structure even after inclusion of the alpha particles, (2) that the Hada-Kennel relations, i.e., Ti/Te <1 and &bgr;<1, are satisfied upstream of this event even after inclusion of the alpha particles, indicating that steepening of MHD slow waves dominates Landau damping, (3) that the Rankine-Hugoniot relations are fulfilled to an absolute difference of <10-5 by employing any possible combination of an upstream with a downstream time-averaged data set, (4) that the MHD evolutionary conditions are satisfied for any solution to the jump conditions, and (5) that the shock speed, the amplitudes, and the relative amplitudes associated with this event are absolutely comparable to those derived earlier for fast mode shock waves within 1 AU. It is shown that this event is an almost quasi-perpendicular slow forward MHD shock wave.