Axle rating is based on the wheels offered with that model truck. The F250 came with 17" wheels and tires rated over 3000 lbs and the F350 came 18" wheels and tires over 3500 lbs. It's the same axle for the F250 and SRW F350 - It does not change until you go to the DRW F350. The difference in codes on your 2001 springs probably spec'ed it as with or without upper overload since you added your upper later.
Here is my 2005 F250 with OEM camper package giving me rear upper overloads and stabilizer bar. I also used the upper and lower StableLoads which allowed me to haul 4000 lbs of camper without excessive droop.
