I believe that the corvus pattern armour (the beaky armour) was introduced a short while after horus and the traitor legions started rebelling. hence that the chaos marines dont have it on their own, I do believe that the traitor legions actualy have forgeworlds or at least forges to make power armour for them and repair their armour in the eye of terror.
I guess not much reason as while the original legions did not have 'beaky' armour the later defectors probably had a bit, like the Astral Claws so there are probably some corvus clad red corsairs about.
Alpha Legion and Raven guard had beakies in thel Heresy, based off of the MkIV with variations like to RG specifications which the AL were somehow managed to requisition/steal. Horus was abke to get lots of resources allocated to his allies rather than loyalists - it is possible that mkvi prototypes made their way to him.
As for no bits that was possibly a concious design decision
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