It's inheritted. Check ShockRifle, which is the super class of ZoomSuperShockRifle's super class. You'll find AttachmentClass=Class'XWeapons.ShockAttachment'. That's what's being attached to players in 3rd person.
You could change the skin properties of ShockAttachment, but that would also change the way SuperShockRifles and ShockRifles looked in third person. Instead, you should probably subclass ShockAttachment, change its skin properties to your liking, and then set the AttachmentClass default property in ZoomSuperShockRifle to that subclass of ShockAttachment that you just made.
Does that make sense?