I am looking for a basic understanding of how this system converts from a 3 wire service drop. A diagram would also be very helpful anyone has one. Many thanks in advance.
4th wire is ground from the exterior service disconnect. 3 wires from the mast/meter PLUS the fourth ground wire then go the interior panel. (the 4th ground wire originates at the exterior service disconnect).
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To put what Rob said in other words: This is a typical 4-feeder SER that feeds the sub-panel inside. The extra EGC (4th conductor) is the uninsulated multi-strand AL and is picked up at the exterior main disconnect right next to (same neutral bar, where they landed the GEC) to the insulated neutral that goes to the panel inside.
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Where is the POA?
In this particular design scheme, the gutter installer likely independently determined that installing the gutter directly over the pesky service drop components was a great idea. 
Do you have a pic of the entire mast?
I had to doctor this image a bit to compensate for the sky.
Here is another.
And here, the wires are in contact with the pole guy wires.
I cannot tell… what is holding/securing the messenger wire to the house?
Must be the gutter. 
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This is the best image there is. I can see a portion of the anchor but it was not visible/covered up by the gutter.
In this area, they are often nothing more than a eye hook bolt/lag with a washer.
Ya, that doesn’t look too good, it shouldn’t be behind a gutter like that.
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This one looks somewhat similar to what you have. The three SECs L1, L2, N come in from the top of the meter side of the enclosure. Turns into 4 wires due to the 2 Neutral bus bar holes on the breaker side of the enclosure.
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I don’t see a service neutral at the main disconnect. I see the two ungrounded service conductors only. And the out the bottom we have the 4-wire feeders to the sub panel, and the GEC.
I assume the meter pan and the main disconnect panel are considered “bonded” since they appear to be attached together. Does that mean the service neutral could be bonded to the meter pan and that is good enough?
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Look at what Yu posted above your post.
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If you look at Yu’s illustration you see the neutral bus bar extends from the meter enclosure through to the disconnect box. It is essentially one box with two compartments and two covers.
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Nice catch Simon. I’ll add that to the list of concerns. 
Perfect. I guess that answers that! Can’t say I’ve seen that setup yet in my area.
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