Dave, you're the man with a copy of the Warhammer 40,000 Compilation; look it up.
Mark 1 was pre-Crusade, marks 2, 3 and 4 were introduced during the Crusade, mark 5 appeared early in the Heresy, mark 6 appeared late on - after Horus' forces started assaulting the solar system (although there was somehow enough time for both sides to distribute it amongst their forces, galaxy-wide) - and mark 7 appeared among loyalist forces during the final seige of Earth.
No date has ever been given, IIRC, for mark 8 armour. Some time between the end of the Heresy and "now" is all you can say. That Lexicanum article is annoyingly poorly referenced, so there's no way of knowing where they got their info.
By the by, the canonical appearance of mark 8 armour has changed; according to Imperial Armour vol. 10, mark 8 armour uses the double-layered shoulder pads from the Company Champion marine in the command squad, and doesn't have the foprearm armour coming down over the back of the hands any more.
The mark 7 suit is just as much a result of Crusade-era development as the mark 2 (albeit with a couple of centuries of technological rediscovery and development), so I wouldn't draw any conclusions from that particular example - which is, after all, missing the usual purity seals, decorational skulls, devotional script, engraving, iconography, etc.
I'd agree that some egions were more ritualistic than others, if it wasn't for the fact that none of the Heresy-era artwork seems to reflect the difference in tone. None of the Horus Heresy novel covers would look out of place in the 41st millennium instead of the 31st. It just looks to me like the artists all missed the essential difference in tone between the settings.
