A question for Death Guard and World Eaters experts: What decals on these sheets are used exclusively for traitor versions of the legions? For the Iron Hands it´s pretty obvious as they have decals depicting the eye of Horus but for the other two it´s not so clear at first glance.
There isn't any, or at least not generally.
The Death Guard, World Eaters, Thousands Sons, and Emperor's Children would eventually each end up marked by their chosen patron Chaos god (and using the iconography of those gods). Otherwise, the various Legions on both sides pretty much kept the same heraldry.
On the Iron Hands, the reason for the few transfers that are cogs with Horusian eyes is that they represent Unit 964. These IH had campaigned with Horus in the Great Crusade, and were later present (and fought as/with the Loyalist Legion) at the Drop Site Massacre. Afterwards, those who survived the Massacre decided to side with Horus since the loss of their Primarch, shattering of their Legion, and their previous experience under Horus' command made this seem like a pragmatic choice to them at the time.