The 1830 edition and the printer’s manuscript disagree here. The question is whether the original manuscript read had came or simply came. If the first case is correct, then Oliver Cowdery accidentally dropped the had while copying from 𝓞 into 𝓟. This error could have occurred because all the surrounding verbs are in the simple past tense. If the second case is correct, then the 1830 typesetter accidentally added the had. The previous clause indicates that these believers already “went forth”, so perhaps the 1830 typesetter expected the past perfect had in the subsequent clause (“and when they had came forth”), even though the first use of forth refers to motion away from Samuel the Lamanite and the second forth refers to motion towards Nephi. In any event, in the 1837 edition the standard past-participial form come replaced the dialectal came after had. For discussion of the use of had came in the original text, see under 1 Nephi 5:1, 4 (or more generally under past participle in volume 3).
Either the past participial had came (equivalent to had come) or the simple past-tense came is theoretically possible as the original reading here in Helaman 16:1. In the original text, we have 18 occurrences of “when X came” and 14 of “when X had came/come” (here I exclude the case here in Helaman 16:1). However, all 14 examples with the past perfect had involve recapitulation; that is, a previous clause mentions an event, and then the following when-clause restates that event. Usually the restatement is a direct repetition, as in the following example:
(Here in Alma 8, both 𝓟 and the 1830 edition have the original nonstandard had came, as does the 1830 edition in Helaman 16:1, rather than had come; in both passages, the 1837 edition made the grammatical change from had came to had come.) Sometimes the restatement is indirect:
On the other hand, in the 18 cases of the simple past-tense came occurring in a when-clause, only three examples involve restatement:
Because of restatement, the use of the past perfect (“had came/come”) is expected in Helaman 16:1. But as mentioned above, such an expectancy could have motivated the 1830 typesetter to add the had here.
One important factor to consider in this analysis is the error tendency on the part of Oliver Cowdery as scribe compared with the error tendency on the part of the 1830 typesetter. Elsewhere in the manuscripts, there is evidence that Oliver sometimes omitted the past perfect auxiliary had. There are two clear cases where he initially omitted the had in the manuscripts:
There is a third case for which it appears that Oliver omitted the had when he copied from 𝓞 into 𝓟. In that instance, the 1830 typesetter supplied the had, but probably only after the 1830 signature had been proofed against 𝓞:
In other words, the typesetter did not independently decide to add the had. He did it only because 𝓞 itself had it (for further discussion of this more complicated case, see under Alma 44:8). In contrast to these errors by Oliver Cowdery, there is one case where the 1830 typesetter supplied the had on his own:
It therefore appears that Oliver Cowdery was somewhat more prone to omit the past-perfect had than the 1830 typesetter was inclined to add it. And since the use of the perfect in when-clauses is a characteristic sign of recapitulation, the original text for Helaman 16:1 probably had the had. We cannot be sure, of course, but at least the odds are somewhat greater that in Helaman 16:1 the original manuscript had the had.
Summary: Accept in Helaman 16:1 the 1830 reading with the past perfect had (“when they had came forth”) as the probable reading of the original manuscript and the original text; in this instance, Oliver Cowdery seems to have accidentally omitted the had from this phrase when he copied the text from 𝓞 into 𝓟; the critical text will restore the nonstandard use of had came in place of the standard had come.