Second portion of response.

Jplayer, I felt like I was going to exceed some word limit or make it too long for members to be interested, but I gave it a try. Thanks for staying with it.

Truck is a probably a 2005 build as it has the OPSS (KOPS) seat interlock, but it only controls the transmission. My understanding is that the B-56 rules to include mast hydraulics went into effect with 2006 year model.
At any rate it is an EPA Tier II truck.
The O2 sensor appeared to be recently replaced due to the overall "new look" of everything about it.
DTC 1241 is cited in our Komatsu SM 300 service manual in Appendix 3, ERROR CODE LIST on page A3-3. DTC 1241 is not listed in the manual section called "Self-Diagnosis and Malfunction Detection Condition" beginning page 5-50 of SM 300. If you have the appendices listed, that is where I found the DTC explained.
I tried resetting the ECU (erase codes) but DTC 1241 returns immediately when engine is started using the new ECU.
If I use the original ECU, DTC 0031 AND 1241 return immediately when engine is started.
Using the SST I can confirm that the codes WILL erase but will be present again when the engine starts again.
I'm not sure but I feel that the relay box and other items are likely chosen by Nissan for use with their K-engines, regardless of whose truck it is in.......even though the actual harness work will probably vary a lot dependent on truck OEM.
I know that the injector driver is made by Nikki.

Thanks to both of you for posting. I will update the thread as I find an answer.
One more thing. I mentioned that this truck had been "worked over" by butchers or RSB artists. The O2 sensor connector on the chassis harness was broken (probably cause they did not know how to unplug it) AND the locking lever on the Injector Driver connector was missing (probably for the same reason.
I salvaged all the harness connectors (just the ones that had not melted) from the other truck in May, so I have used those parts to restore the items on this truck