The Herschel great-grandparent is a specific three-tick predecessor of the Herschel, commonly seen in Herschel conduit collections that contain dependent conduits. In some situations it is helpful to display the input reaction in this form instead of the standard Herschel form.
Dependent conduit inputs are catalyzed by a transparent block before the Herschel's standard form can appear, and before the Herschel's first natural glider is produced. This means that these conduits will fail if an actual Herschel is placed in the "correct" input location for a dependent conduit. Refer to F166 or Lx200 to see the correct relative placement of the standard transparent block catalyst.
Almost all known Herschel conduits produce a Herschel great-grandparent near the end of their evolutionary sequence. In the original universal set of Herschel conduits, Fx158 is the only exception.