Some more detailed information about the actual hardware used:
LED Boards... There are 10 of these (the lost world uses 12 though I believe). 5 across the top and 5 across the bottom. I think the 12 sensor setup used an addition sensor on the left and right sides.
The lost world used a version with two verical LEDs, then with HOTD2 they changed it to 2 horizonal LEDs with a little hood:
at some point later they changed again to a version that uses 4 LEDs and no hood:
From what I understand they're all intercompatible, you can have a mixed collection of these boards in the daisy-chain and it still works fine.
I do believe the order of the sensors in the chain matters though. It starts one in from the bottom right of the screen then goes around clock wise ending on the bottom right.
Sensor Board... this is littereally the only thing inside the gun aside from the trigger switch
there are a few different revisions of this, I've seen them with longer or shorter lenses depending on the gun casing they're going in, but from what I understand you can use any sensor with any Gun I/O revision, the pin out is the same and the Gun I/O doesn't care. It has a trigger signal wire, and a ground reference and the rest of the pins are for the sensor.
Gun I/O Board:
Many different revisions of this board, the early ones for Lost world need 12 sensors, but the rest only need 10. They all have the same connectors, a 2-pin 12V power connector, a connector for each gun, a connector for the LED board daisy-chain, and an 9-pin output connector for the analog X and Y on each gun as well as a trigger and "screen out" "buttons" as well as a ground reference.
The 3-pin CN1 connector doesn't seem to be used though mine has a connector that jumps pins 1 and 2 so maybe it's some kind of external jumper setup.
Gun Protect Board:
This connects to the gun connectors on the Gun I/O and then the guns plug into this. They started using this around 2003 and I believe they sold these board separately for people who wanted to retrofit them to older games. Newer Games use a fancier "fuse board" that also includes fuses and protection circuits for the recoil solinoid.
this board is technically not required, it's recommended that you run one if you don't want your ish to break.
If you want any detailed pictures of the boards or IC numbers let me know.