Different part numbers are not the same parts. The brown dot springs give the ahc a lower pressure, making it easier to carry a bit of load and giving the globes an easier life.
You can get 48231-6A730, 48231-6A740, 48231-6A750 (H=455mm, pink), 48231-6A770 H=485mm, purple) or 48231-6A780 (H=500mm, brown). All will fit, but having slightly different lengths and/or rates. You can have 2 different ones with the longer/stiffer on the left, to help the TBs balancing the truck, or just leave the extra weight on the left to the TBs. Both methods are used.
I guess it comes down to what you can get hold of, as some might be out of stock, I've heard.