I saw that and actually might use one on another motor that is a 360LA which that is for. 5.9 Magnum has a different balance.
The weight the PF listed is the only one that I can find that is for 5.9 Magnum only.
McLeod sells a kit with several different weights. One of them is for the magnum.
As far as I can tell, the one that PF recommended is a copy of the McLeod weight. It may be an actual McLeod weight from their kit. I suspect the seller is buying the kits from McLeod and selling the individual weights at a great profit.