Oliver Cowdery initially wrote “in the knowledge of him” here in 𝓟, then inserted with the same level of ink flow the additional words of the glory, thus giving “in the knowledge of the glory of him”. Since either reading is possible, the original manuscript probably had the phrase “of the glory”. Oliver’s eye probably just skipped from the first of to the second one (“of the glory of him”) as he copied from 𝓞 into 𝓟. This same phraseology (“the knowledge of the glory of God”) is found near the beginning of the preceding verse (Mosiah 4:11). Eight manuscript lines separate these two instances of “the knowledge of the glory of X”, so it is probable that Oliver did not notice the identity in phraseology and was therefore not prompted by the first instance to consciously emend the second one.
Summary: Retain in Mosiah 4:12 Oliver Cowdery’s corrected reading: “the knowledge of the glory of him” (which was probably the reading of the original manuscript).