Both manuscripts omit any preposition before the phrase “some future day”. The 1830 typesetter added the preposition at. The question is whether there really needs to be a preposition in front of “some future day”, especially since the sentence is in the passive and, as a result, “some future day” can hardly be misinterpreted as the direct object of “brought forth”.
Elsewhere we have two examples of adverbial phrases containing the word future, and each is headed by a preposition:
The second example suggests that the 1830 typesetter could have inserted the preposition in rather than at for Enos 1:13 (“that it might be brought forth in some future day unto the Lamanites”).
There are a few other adverbial phrases involving some and a period of time that are headed by a preposition:
Even so, English has some idiomatic adverbial phrases of the form “some ” for which there is no preposition, as exemplified by the following instances in the Book of Mormon:
Of course, in both these examples the word future is not present.
The Oxford English Dictionary has 21 citations of “some future ”, but in each case there is a preceding preposition (19 with at and 2 with on):
Thus there is no evidence in the OED for the earliest usage in Enos 1:13.
Even so, the earliest reading in Enos 1:13 does not seem especially difficult. Although unexpected, this nonprepositional usage is possible and may very well be intended here. The critical text will therefore restore the earliest reading in Enos 1:13 (namely, “that it might be brought forth some future day unto the Lamanites”).
Summary: Restore in Enos 1:13 the original adverbial phrase without the preposition at (or in) since the adverbial phrase “some future day” can occur without a preposition.