Looking at the models, I'm noticing the Eagle and lightning bolts on their chests - markings of veterans from the conquest of Terra by the Emperor. Were these Imperial Fists originally Terran born and fought through the Conquest of Terra, the Great Crusade, and the Horus Heresy? These are some seriously veteran guys.
I wish I could pit my Iron Warriors against them Win or lose, it's a great narrative setup. Terran-born Primaris, veterans of countless wars, against the firstborn Legionnaires who turned traitor ten millenia ago.
I'm also wondering this. Because the Raptor Imperialis is a VERY distinct icon, and the Adeptus Astartes still know what that symbol means.