well I can give you my 2 cents as owning a M880 and M890 for many years and also seeing them haul (some ungodly bulky loads and trailer loads) in the military when they were new .. It is my understanding that although the rear axles share the same model number, the M's supposedly have a heavier rear axle than the civilian trucks, thus a higher load capacity rating.. Consider that some Jeep CJ's had dana 44's and 60's, and you know how light those axles are in comparison to a 3/4T truck with same model numbers ... Dana model number does not determine weight rating.. As for the springs you see, the M880/890-series rear springs are usually not any different than the 3/4T civilian trucks, thus you get an equivalent 3/4T ride empty. BUT if you get underneath your truck and look , you should see a pair of what we called "load blocks" or "limiter blocks" between the frame and spring set .. As the load increases, the frame eventually hits the blocks and thus your springs are no longer carrying the load. This allowed more weight to be piled on without bending the springs downward any more.... The only negative experience anyone I know had ever had with these blocks is when a load puts the frame down onto the blocks just a little bit. Then, when every bump or hump or railroad track causes the truck to lift up, it then comes back down on the blocks with a big "thud!" that you swear is gonna break the truck in two .. If the truck is on a rough and bouncy road, it is like getting hammered "bang! thud! .. thud... thud thud thud,... wham!" The key is to either run it empty or on up to a 3/4T load to keep it off the blocks, or load it to the max to keep it down on the blocks .. But at 5/4T, the truck will definitely sag in the rear, be light in the front, and will want to sway on corners .. I rarely put more than 3/4T in either of mine, but they both have handled 5/4T loads when tasked. It's the rear tires that I worry about with loads like that but I supposed I had gotten spoiled by the security and stability of dually pickup trucks ..
I don't have time at the moment to confirm the axle difference with reference to a parts manual or online, but that is where you can look for yourself ..Welcome to the 1970's power-wagon world !