The (partial) definition is there in the Coordinates header: Lo 31. That's a South African coordinate reference system (CRS). It might be based on the Cape datum/geographic CRS or the current Hartebeesthoek94 CRS. We do a weird workaround to support these coordinates because the axes are south-west (or maybe west-south order) rather than our default of east-north.
Try entering them as they are using projected coordinate systems-national grids-south africa-Hartebeesthoek94 Lo31 and see how they work. If they appear to be flipped, try the (E-N) version instead.
Melita