After reading Deliverance Lost I've been thinking about the power armour types and the fluff attached to them. Marks II to IV seem to be pretty straightforward as they are all Imperial mass-production equipment, but then it becomes a bit harder:
In Deliverance Lost we are told that Mark V "Heresy" -pattern was actually never produced as such. After Isstvan the Raven Guard techmarines basically build new suits out of Mark II, III and IV remains, stapling the pieces together with special rivets they had invented. After coming back into contact with loyal Mechanicum and Imperial Fists this "quick-fix" style spreads further and Mechanicum decides to give designation Mk V to all stopgap and custom power armour designs. At the same time Mechanicum has actually started producing a new power armour model, Mark VI "Corvus", which is named in honour of Raven Guard and their primarch and the first batch is given to the new Raven Guard recruits, the "Raptors".
Okay, now the question is: If Mark V is basically just putting together old armour pieces with new rivets, then where does the distinctive Mark V helmet come from?