Having had many 555 drivers on my bench wanted to share a detail that illustrates the 3 variations of the pole plate. To anyone who doesn’t know the pole plate is a critical part and what dictates the magnetic field around the voice coil. The earliest one is 1st image and yes is a 555w. Judging it seems to me that this version is more costly production method. As seen one has the field coil removed others. The wedges you see are hardwood and are a very simple yet effective way to center the field coil.

All fun…