So, you should be able to get specific head customisations from Zarpedon/The Sentinel (e.g. Void Rocker for Wilhelm, Tech Ninja for Athena, …)
I have been farming both of them a couple of times now and they dropped the according customization for Athena multiple times and another one for Wilhelm aswell… only that it was the Lunar Knight and not Void Rocker. (Tested this several times, Void Rocker never dropped but Lunar Knight did more than once.)
Could it be that Lunar Knight somehow replaced Void Rocker in the lootpool? Has anyone else experienced this?