I have a
b2620 with the same manuals, some of the writing is generic between the models, the way I read that is the forks are an option on the la504, not that it was an option for the tractor.
I've found the manuals are lacking a bit in translation at times and there can be references to equipment that's not for your tractor in the manual.
As for the relief adjustment, step one is get a 3-4000 psi gauge, and some fittings so you can attach it to one of the lines for the loader (I can't remember the exact fittings needed) or buy the kit from someone, I believe bxpanded sells one that'll work.
The relief is located on the loader valve under the right side of the floor pan, there is a hex nut cap that hangs down that needs to be removed and the relieve valve, spring and shims will be inside.
You will lose a little fluid and in my case the valve decided to slide out shortly after I removed the cap so I would suggest a catch pan and somthing like a paint filter in case any of the shims try to escape (happened to me).
You can use washers or you can buy shims from mcmastercarr, kubota, or bxpanded I believe. I made my own on my lathe.
I don't have any pictures but can try to take some next time I have the tractor out. There is some good info online and a chart for what thickness shims will increase the pressure a certain amount.
I finally adjusted mine last year, was a little low but not much, I searched on the subject and it was pretty straightforward, but I also have some background in mechanics and hydraulics.