Here we have another example of a participial clause that is never completed. For similar examples (involving knowing ), see the discussion under 1 Nephi 3:17. Here in 1 Nephi 4:21, the author (Nephi) shifted the focus of his narrative when he decided to explain why Zoram mistook him for Laban. One could interpret this unfinished construction as a Hebrew-like participial clause with an unstated be verb (“for he was supposing me to be his master Laban”).
Oliver Cowdery himself edited this participial clause. After copying 𝓞 into 𝓟, he decided to emend the nonfinite verb form supposing to the finite supposed. This change was done with heavier ink flow, which implies that the grammatical emendation was not immediate. In this instance, 𝓞 is extant, so we know that the original text read supposing (spelled as soposing). And the example in Enos 1:1–2 (discussed under 1 Nephi 3:17) shows that the text allows such stranded participial clauses.
Participial clauses using supposing are found elsewhere, but they are all completed, as in this nearby example:
Summary: Restore the original participial clause in 1 Nephi 4:21 (“and he supposing me to be his master Laban”) since 𝓞 reads this way and such unfinished clauses are occasionally found elsewhere in the text.