I am much less certain of the use of the oblong face on the lower arm. The lower marker appears to have been used solely to designate the signal as an interlocking signal. This is not explicitly stated in the rulebook, but follows from the aspects with which this marker is associated. AMTK Amtrak acquired a major legacy of PRR signalling. Current AMTK yellow is darker in hue than previous PRR amber.
Colorized signals lose their middle lamp on the upper arm.
Liberal use of bracket posts. Some roads used Advance Approach also, some used Approach Medium in identical settings, and some did not use this concept at all. Stop signals are found only at interlockings or other such controlled locations. The second word provides block occupancy information, clear indicating at least 2 free blocks, approach indicating no more than 1 such free block. Route-signalling aspects, because of their lack of need to provide speed information, the number of aspects is greatly reduced.