DP page 740 and 741 has has willem as c3d4 married to Martha Louisa Erasmus.
They do not have a child Marthinus Lourens.
however
c3d6 Joachim Johannes married to Elizabeth Johanna Erasmus has e6 Marthinus Lourens baptised 10 Feb 1793.
Ben Cilliwers page 476 also agrees with the DP notation c3d6e6