Close reading and inspection of the Horus Heresy rule books reveals a number of hidden gems and "easter egg" style gifts to the gamer. The first one of these is the Anvilus Pattern Backpack that one of the Death Guard can be seen wearing in Betrayal. Today, I wanted to note another one from Betrayal:- the Sarum Pattern Respiration Mask, pictured below.
The story in Betrayal goes that the World Eaters managed to form an alliance of sort (such that the World Eaters are capable of) with the Sarum Forge World having been deployed there and seen serious action. As a "reward", they got supplied with a new pattern of respirator. The nice part about this particular helmet is its visual similarity to those included in the space marine boxed sets. Although clearly a much later pattern, the Mk VII Aquilia Armour is probably the one that is closest to the Sarum pattern. That said, there are some subtle differences. But at a distance and at first glance, both the Mk VII helmet and the Sarum one are almost the same. Therefore, the World Eaters players should feel free to use this helmet in their conversions. And probably Alpha Legion as well - because they're Alpha Legion!