Here Oliver Cowdery initially wrote the singular labour, then changed it to the plural labours by inserting an s inline at the end of labour. This correction was probably almost immediate since the level of ink flow remains unchanged. Elsewhere in the Book of Mormon text, we have six occurrences of “labors of” but none of “labor of ” (including cases where labors is conjoined with a following noun); most of these occurrences are found in the phrase “to glut (oneself ) upon the labors of someone else” (or its equivalent):
Summary: Retain in Mosiah 7:16 the plural labors in “the labors of their journey”; Oliver Cowdery’s correction in 𝓟 is nearly immediate and is supported by usage elsewhere in the text.