Dan, they are 2 different kinds of projectiles. The all balck one is a solid Armor Piercing projectile, exactly like the drill cartridges I have. The other is a High Explosive round that is hollow and filled with TNT designed to explode on impact. They are both correct for the 57MM, just 2 different loads. I have one round that looks like your first picture with the windscreen on it, however it is all steel to the tip. Another variation. Need to get my book out to identify the nomenclature for each, but the A P rounds did not have a windscreen.
Hope you are doing well. Taking the M18 down to Albany this weekend for the full auto machine gun shoot and am going to live fire the main gun along with my live .50 . Gonna be a blast!! (Pun intended)