Here in the printer’s manuscript, Hyrum Smith initially wrote writing in 𝓟 (accidentally spelled as wrting), but he immediately crossed out the word and wrote inline written word. The original manuscript undoubtedly read this way given that there are no other occurrences in the text of the noun phrase “written word(s)”. There are, on the other hand, occurrences of the noun writing preceded by the indefinite article: “a new writing” in 1 Nephi 16:29 and “a writing of divorcement” in 3 Nephi 12:31 (as in Matthew 5:31). There would have been no motivation for Hyrum to emend the text here in Mosiah 29:4 except to correct his copying error.
Summary: Maintain Hyrum Smith’s immediately corrected reading in 𝓟 for Mosiah 29:4: “yea even a written word sent he among the people”.