Makes sense, after you own a 6 stud arms, stock parts look like toys.. lol I'm upgrading to ARP studs too, should be nice & strong.
Marlin made them when the six stud knuckles were first being designed by Marlin and Chris. When Chris bailed and went to TG he said the design was all his own and took it with him. Instead of fighting about it Marlin just let it go. There are very few sets of the Marlins out there, I just happened to get a set when I broke one of Marlins arms and he replaced them for me since they also would work on stock knuckles.
