Space marine armor variants

[Lordgimpet]

Hey guys just a quick one
what version of armor did the marines wear just before or at the very start of Horus's betrayal?
Ive talked to a few people in person but no one could agree on one variant or even a couple. So I put it out there in the web :)

[Gotthammer]

Mks 2, 3 and 4. Mk2 is the first fully sealed suit, used by all legions at the beginning of the Crusade. Mk3 is the heavier 'assault' version of Mk2 - it is not a replacement but a counterpart intended not for open warfare, but close in fighting such as boarding actions or tunnel fighting.

Mk4 was to supercede both 2 and 3 (though 3 would likely be kept around for special missions), being improved in protection, maneuverability and fuel efficiency (Mk3 would strain power supplies and be very hot to wear). Mk4 was issued just before the Heresy broke out, mostly to the Legions that went traitor (likely due to Horus' meddling).
As a result spare parts were scarce, and the technology required very high tolarences, so Mk5 was built as a stop-gap using lessons from Mk4, but parts from Mks 2/3 where available, and simpler bitz (the helmet is a spin off from the terminator project). There's no precise mention of when exactly it appeared, but it would have been fairly near the begginning I'd imagine, the first two-three years maybe.

Mk6 came online at the very end of the Heresy, when the traitors were in the Solar system, and Mk7 at the very very end, around the time the traitors took the moon.