The occurrence here of skin without the indefinite article a seems strange. The text is obviously referring to animal skin; but without any modification for the noun skin, one tends to initially misread skin as referring to the Amlicites’ own skin. Usage elsewhere in the text suggests that the indefinite article a is missing before skin in Alma 3:5:
Except for the a, the phraseology in Alma 3:5 is virtually the same as in Alma 43:20: “but they were naked save it were a skin which was girded about their loins”. This usage with the indefinite article is also supported by the language in Mark 1:6 in the King James Bible: “and John was clothed with camel’s hair and with a girdle of a skin about his loins”.
There is also evidence that scribe 2 of 𝓟 sometimes omitted the indefinite article a:
In the last two instances, the 1830 edition was set from 𝓞, not 𝓟, thus providing additional support that the a was in the original text for both these cases. Further, 3 Nephi 22:6 is a quotation from Isaiah 54:6, which has the a before wife in the King James Bible. Clearly, scribe 2 may have omitted an a in Alma 3:5. In accord with the consistency of the text elsewhere, the critical text will emend the text by adding the indefinite article a before skin in Alma 3:5.
Summary: In accord with usage elsewhere in the text, emend Alma 3:5 to read “save it were a skin which was girded about their loins”.