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I'd think you would need to first establish as what counts as "official Horus Heresy lore". The answer to this question will undoubtedly depend on the year when a person got into the 40K hobby.

The marines in 1st Edition Space Marine were MkVI. At that time, there were only 1 model for all Marines, you used a coloured flag on the models base to denote if a unit was Assault, devastator, tactical etc.

The expanded model range, with the combo kit featuring terminators, assult marines, bikes etc came later. This combo kit had MkVII as tacticals and MkVI as Assault