Oliver Cowdery initially wrote the expected Book of Mormon phrase “of gold and of silver” here in 𝓟. This phrase, in fact, was used earlier in this chapter: “and they did have an exceeding plenty of gold and of silver and of all manner of precious metals” (Helaman 6:9). Here in verse 31, Oliver corrected his initial reading to “of their gold and their silver”. The correction seems to have been virtually immediate since there is no change in the level of ink flow for the crossout of the repeated of nor for the two instances of their that were supralinearly inserted. The original manuscript, not extant here, very likely read “of their gold and their silver”.
When gold and silver are conjoined in a prepositional phrase headed by of, if a possessive pronoun modifies both gold and silver, then sometimes the of is repeated, sometimes not. Besides the case here in Helaman 6:31 without the repeated of, there are three cases with the repeated of, but there is one without (marked below with an asterisk):
So the of does not have to be repeated, and the corrected reading in 𝓟 for Helaman 6:31 will be maintained.
Summary: Accept in Helaman 6:31 Oliver Cowdery’s correction in 𝓟 to “idols of their gold and their silver”, where the possessive pronoun their is repeated but the of is not.