So wait - early Mark VII during the (late) HH?

[Kol Saresk]

And since we know that the Raven Guard are in cintact with Terra and that Corax is using the foundries of his own homeworld to produce new sets of Power Armour, there is even another alternative that is a Raven Guard-specific modification that Terra learns of and incorporates into its design of the Mk VII. Much as how we know the Mk IV Sarum pattern helmet used by the World Eaters gets used as a precursor to the Mk VII helmet.

 

There is no one right answer and just because it isn't the right answer of any specific individual, doesn't mean it's wrong.