Use the file I had w/ the descriptions, I SAID, colors can change. They are not consistent between ALL years/panels/engines. They can even change between what Seaward and Universal used, and the harness CTY ran between those two.
The RED wire on I is likely your fuel pump, powered whenever the key is on.
OK, now I see the #10 WHT wire on S now - that powers the coil on the preheat relay down below. It used to run to the glow plugs (might be called out as gray?)
The #10 RED on B is the power feed from the starter solenoid B terminal (UNFUSED!!!!)
The yellow wire from the Start PB is the "S" wire circuit to the starter solenoid. It's light, needs to be #10 gauge.
The #10 orange wire from the harness -- where does that terminate on the panel (just unused and hanging)? After you remove the gummy bear plug, you can substitute that #10 orange wire for the "S" circuit.
On the schematic basically, A is I. The start and preheat were separate push buttons -- BUT REMEMBER the M-25/XP schematic is for the Universal panel, NOT your (or my) Seaward panel that CTY installed. There were 3-4 different Seaward panels produced for the M25 and XP. I have a key start and separate preheat toggle sw. So no single schematic covers all the permutations.
PO obviously powered something else from I - have no clue. Trace it?