It involved "an incident in the life of Oliver Cowdery" that was not documented in any writing we have from Oliver. Oliver explained that he and Joseph actually visited the depository of Nephite records in the Hill Cumorah in New York.
President Young said, "I relate this to you, and I want you to understand it. I take this liberty of referring to those things so that they will not be forgotten and lost."
And yet today, sadly, these things have been forgotten and lost by most members of the Church.
Because President Young reaffirmed the teaching of Letter VII that the Hill Cumorah is in New York. This contradicts the Mesoamerican/two-Cumorahs theory (M2C) so LDS intellectuals and their followers don't want members of the Church to even know about it.
And when people do learn about President Young's teaching, the M2C intellectuals dismiss it as relating a vision of a hill in Mexico.
When he wrote Letter VII and the other letters, Oliver Cowdery mentioned that he was relying on original documents in his possession. We don't know what documents he was referring to, but it could have been the notebook he kept of everything Joseph told him. Or it could have been journal entries, letters, or notes that have since gone missing.
We'll see President Young's teaching below, but first let's look at how our M2C intellectuals dismiss it.
FairMormon has an article on this here:
They quote part of Brigham Young's sermon, omitting the part about other witnesses to the site and Brigham's emphasis that he did not want this account to be forgotten and lost.
Then they make this awesome statement:
Of course, nothing in President Young's discourse states or implies he was describing a vision. Several other contemporaries of Oliver Cowdery mentioned his visit to the depository of Nephite records. And, of course, in Letter VII President Cowdery reaffirmed that the New York hill was the site of the depository.
To justify their rejection of Letter VII, M2C intellectuals rely on a comment by Heber C. Kimball about the event in which he spoke of a "vision" that Joseph and Oliver had when they entered the Nephite depository.
Does that term really mean the experience was purely spiritual and not physical?
It's a poor argument because Joseph Smith also used the term "vision" to describe the physical experience of Martin Harris seeing the plates as one of the Three Witnesses.
"We accordingly joined in prayer, and ultimately obtained our desires, for before we had yet finished, the same vision was opened to our view; at least it was again to me, and I once more beheld, and heard the same things; whilst at the same moment, Martin Harris cried out, apparantly in ecstacy of joy, “Tis enough; mine eyes have beheld,” and jumping up he shouted, hosanah, blessing God, and otherwise rejoiced exceedingly."
This is another case of the M2C intellectuals undermining the teachings of the prophets so they can justify M2C.
Instead of repudiating the teachings of the prophets, the M2C intellectuals would do a wonderful service by embracing those teachings and repudiating their own theories instead.
Here's a link to a discussion of President Young's sermon about the depository of Nephite records in the Hill Cumorah in New York.