Hey there,
while I don't have the IG codex, I was wondering what the Imperial Guard was like during the Horus Heresy era - I mean, were there very marked differences to what we're seeing today? What was the most commonly seen heraldry (uniforms, banners and such), the equivalent of 40k Cadia more or less? I guess the attrition tactic stayed the same, but what about the equipment - were Leman Russes mainstay tanks at that time already?
Also, the Solar Auxilia the Forge World is producing - are they the IG of those times or the IG of the time of the Great Crusade?