In the book Fallen Angel, lion El Johnson gives Pertuarbo 3 Ordinatus Siege Engines telling him to put them to good use at Isstvan.
Have these reappeared yet later in the series, have we seen the Iron Warriors use them at all?
It's mentioned again briefly in book 20 "The Primarchs" at the start of the short story "The Lion".
Lion El'Johnson appears much less trustful than before and it's mentioned:
Trust had been a scarce commodity for the Lion before Horus's betrayal and even then he had been taken for a fool. Perturabo had used his status as a brother to trick the Lion, taking control of the devastating war engines of Diamat under the guise of alliance, only to turn those weapons against the servants of the Emperor. The shame of being so manipulated gnawed at the Lion's conscience and he would never again accept the simple word of his brothers.