Here the corrected text in 𝓞 reads “a descendant of Laman’s”. Oliver initially misspelled the name Laman as Lamman. He made the same mistake when he wrote the name a second time, in the immediately following verse:
Alison Coutts points out (personal communication) that the misspelling Lamman here in Alma 55:4–5 may represent a mispronunciation on Joseph Smith’s part of Laman as /læmßn/ rather than /leimßn/. Perhaps Joseph didn’t realize here in Alma 55 that the text was referring to the original Laman, the oldest son of Lehi. The name Laman is fairly infrequent in the preceding part of the book of Alma, occurring only four times:
In each case, the reference is to both Laman and Lemuel (and also in three of the cases to the sons of Ishmael). But here in Alma 55:4–5, some 33 manuscript pages after the last reference, the text suddenly refers only to Laman, which may have momentarily confused Joseph into thinking he had a new name here.
In any event, for both instances of Lamman in 𝓞, Oliver Cowdery crossed out the incorrect Lamman and supralinearly inserted Laman. But in the first case of the corrected spelling for the name (in verse 4), Oliver Cowdery initially wrote Lamam supralinearly, which he then corrected to Lamans by overwriting the final m with ns. This final form, Lamans, stands for the possessive form Laman’s. In general, Oliver did not use the apostrophe when writing down possessive forms in the manuscripts (for this point, see the discussion regarding the phrase “three days’ journey” in the 1 Nephi preface). The possessive form Laman’s is clearly intended in 𝓞; the only question is whether it is textually correct. When Oliver copied the text from 𝓞 into 𝓟, he apparently thought otherwise since he omitted the possessive s and wrote “a descendant of Laman” in 𝓟. The printed editions have continued with this reading.
Under Alma 46:24, I list a number of cases in the manuscripts where Oliver Cowdery wrote— sometimes initially, sometimes finally—instances of the double genitive (namely, noun phrases of the form “X of Y’s”), such as “a descendant of Laman’s” here in Alma 55:4. For each of these cases, the critical text will basically follow the final manuscript reading in determining whether the original reading was an instance of the double genitive. As discussed under Alma 46:24, there are four cases of original double genitive in the manuscripts.
Elsewhere in the original text, there are 28 instances of “a descendant of X”, but no others of “a descendant of X’s”. But for each of these 28 instances, the intent of the phrase is to declare someone’s lineage, as in Alma 54:23: “I am Ammoron and a descendant of Zoram”. Here in Alma 55:4, on the other hand, Moroni is hunting for someone who is a descendant of Laman (that is, a Lamanite). Thus there is a systematic difference in narrative purpose between this example in Alma 55:4 and all the other examples of “a descendant of X”. Since the corrected extant reading in 𝓞 for Alma 55:4 will work, the critical text will accept this particular instance of the double genitive.
Summary: Restore in Alma 55:4 the corrected reading in 𝓞 that takes the double genitive form “X of Y’s”, namely, “that perhaps he might find a man which was a descendant of Laman’s among them”; as noted under Alma 46:24, there are three other examples of the double genitive in the original text.