Model Board Symbology Questions

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Hi All.

I have a photo of Circleville Tower’s model board and have some questions about the symbology/nomenclature used. See the attached photo for references.

Signal 6794 - The symbol for this signal has what appears to be a lower arm. Signal 6778 does not have this. What is it?
Signal 6R is similar in appearance to 6778, except that 6R has a “hollow” quadrant. What does this difference indicate? Is it a home signal compared to an ABS signal perhaps (6R seems to be within the scope of the tower, based on dad’s recollections).
Signal 6794/6778 are numbered based on milepost; what does 6R, 10L and similar mean? That they are within the interlocking plant?
What do Triangles 9 and 17 on the Old Main indicate (for reference, the old SV main was used to switch local industries, and is still in place today)
Similarly, does the R / L simply indicate which side of the track the signal is on? If so, why is this needed vs. just having unique numbers?
Is there any significance to the red/yellow alternating block colors beyond making it easier to see where each starts and stops?


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