It's been a long time since I worked on the SP, but I think you may be referring to indicators next to certain switches (which may or may not have been electrically locked...I don't recall that for certain) that showed block occupancy in either direction...in other words, whether something was coming. As I recall, those may have been in place where a secondary track entered the main at a point where there were no block signals close by. They had little semaphores inside the round fixture.

Though I worked in both CTC and ABS territories I don't recollect whether the indicators were used in one or the other or both. Hopefully someone can add more details.