Remus Ventanus wrote:So a marine in mk 4 could choose to wear a mk 2 or even mk 6 helm? And I've also seen helmets on the covers that belong to no mark at all. Not sure if there legion specific or not
Yep, that's about how it goes. Usually, it will be a case of armour being passed down and acquiring almost relic-status, so for example if Sergeant Bob won great deeds in his Mark 6 chestplate, then it would be passed down to his replacement that he might honour his fallen predecessor.
Once the Heresy really kicks off, you also get all sorts of supply issues, so you'd get marines cobbling together the best suits they could from what was available.
As for legion-specific bits, there were a few deviations for specific units (like the Emperor's Children Palatine Blades or the Iron Hands Medusan Immortals) and the Ultramarines produced their own armour mark after supplies from the Imperium were cut off.
Artistic license does play a part as well, of course.