Mark VI availability during the HH?

[Master Sheol]

I believe you're thinking Mk VII there

No

According to fluff MkVI was mass produced in the last months of HH and distributed to the legione defending the solar system (this was retconned later with RG having access to experimental MkVI before)

MkVII was introduced in the last weeks of HH when the Siege of Terra started

 

MkVI was produced in Moon cause Mars was in civil War of mechanicum vs dark mechanicum

MkVII was produced directory in terra cause Moon fell to Horus fleet

 

So MkVI should be only for RG, BA, IF, WS before heresy end with very Little numbers to other legions (experimental ones) while MkVII is BA IF WS only and Just during the Siege of Terra

[Ryltar Thamior]

Given the number of colour plates etc. from the various Black Books featuring Marines of seemingly almost all the Legions in Mk.VI, it seems prima facie obvious that via whatever means [early supplies, experimental/prototype allocations, intercepted armour meant for other Legions, directly pilfering it off the corpses of the foe, coming up with stopgap plate that consciously mirrored Mk.VI, whatever the Alpha Legion were doing, etc.], certain amounts of it ended up in the hands of just about everybody. 

As has already been stated, ,we have direct confirmation of significant quotients of it for the XIXth and XXth, with the association of it with the XIXth being so strong as to render it "iconic" even beyond the fact it was already "eponymous", so to speak. We *also* have interesting confirmation for it being parcelled out to certain smaller-sized formations such as the Knights Errant [predictably] and the kill-team operating under Meduson which appears at Dwell. This last one, in particular, is rather curious as it either implies a hitherto unknown degree of supply co-ordination to Meduson's force from forges in direct contact with Mars/Terra , or that Meduson's contingent were capable of producing/refurbishing/upgrading the suits in question themselves, or that they perhaps got these as a result of their 'shattered legion' association with Salamanders and Raven Guard personnel post Isstvan V. 

We *also* have confirmation of a relatively small number of suits , albeit of 'pre-'Mk.VI plate [so possibly without hte advanced sensors and other innovatoins that made it such a good 'stealthing' armour so beloved of the Raven Guard's modus operandi] turning up in the armouries of both the Iron Warriors and the Salamanders; albeit with neither legion finding it particularly favourable due to their preference for heavily armoured 'endurance' based slugging matches. And as applies my own loyalist Iron Warriors, this is part of how I justify them being in Mk.VI to a man - because they were being punished by their Primarch :P 

However, there's one set of sources I don't believe have been parsed yet ... and those're the old Index Astartes articles [some of the best stuff ever to grace the pages of White Dwarf imo]. Now, admittedly, they're presented somewhat as being post-facto, and with various  gaps in the knowledge resultant from the occlusion of time and bias [a great example of which being hte contrasting accounts of the Iron Cage incident in the IVth and VIIth legion articles] ; however they're still an interesting and somewhat useful source. I raise these becuase the colour plates at the front of those articles, depicting Crusade/Heresy-era colour-schemes alongside their 41st millennium descendents, tended to depict Mk.VI armour in most cases [albeit not all - the Sons of Horus for instance, has a chap in what appears to be Mk.IV ... while, perhaps more interestingly, the Imperial Fists Heresy-era Marine appears to be in of all things, *Mk.VIII* ]. It's certainly possible that this is explicable away as a result of the in-universe archivist getting things wrong or whatever, but particularly in light of hte number of supposedly Heresy-era relic armour-pieces spread out across the First Founding chapters that are Mk.VI, I'm not so sure. 

I also suspect that the situation of a lot of 'modern' Chaos Space Marine armour being clearly at least partially Mk.VI derived further suggests widespread adoption/incorporation of it by the close of the Heresy.