May 2008 A document scan recently posted, "Preliminary Orders and Notes on Communications," helps explain an unusual device shown in a few chaser photos.
This photo of SC 90 shows the device, an add-on light unit attached to the bearing indicator.
The document indicates that, at the time the document was circulated, recognition signals were being sent using the lights on the bearing indicator, and orders had been given to install twin lights instead of the single lights at the tip and feather of the bearing indicator. Very few photos show this device. My guess is that the idea of using the bearing indicator for this purpose was abandoned in favor of using the mast/truck lights (which were used for various different kinds of night time signals).