In the printer’s manuscript, Oliver Cowdery initially wrote “brought out of prison” without the forth; almost immediately Oliver supralinearly inserted the forth (there is no change in the level of ink ﬂow). This change is probably not due to editing but instead represents a correction to the reading of the original manuscript, especially since either reading is possible. In Alma 37:25, we have another example of “to bring forth” where the forth is not required:
On the tendency of the scribes to omit the word forth, see the discussion regarding the clause “their cry shall go” under 2 Nephi 3:20.
Summary: Retain in Alma 20:28 Oliver Cowdery’s virtually immediate insertion in 𝓟 of forth (thus “the brethren of Ammon was brought forth out of prison”).