This lesson was moving and gives students a personal look into history. Your UDL principles are clearly listed in your lesson plan by offering alternatives and/or additional support to students with those needs. Giving the students the ability to share their letters and reflect on the horrible past that the postcards depict will give students a sense of understanding of what actually occurred in terms of race relations and what that means for them today. I agree that the age level for this lesson should be no younger than 15. The accompanying Google Docs lesson plan offers many different activities that can help students understand the objectives you set for them.