Here in the printer’s manuscript, Oliver Cowdery initially wrote “in the land of the Lamanites”, but almost immediately he supralinearly inserted “borders of the”. Undoubtedly, the original manuscript read “in the borders of the land of the Lamanites” since there is nothing actually wrong with what Oliver initially wrote in 𝓟. The corrected reading also agrees with the text’s earlier reference to the separation of these missionaries at the beginning of their mission:
Summary: Accept in Alma 21:1 the virtually immediate correction in 𝓟: “in the borders of the land of the Lamanites”.