Running this down from the beginning of Myst V:
Descent - Plasma 1 content that was scrapped between DIRT and Uru Live. Was later reworked for Myst V.
Direbo - Possibly a purely Myst V age. The Urwin located in Direbo is actually much much larger than the one in Negilahn. About twice the size. It's likely that the original idea was to have a Urwin appear in Direbo, walking along the perimeter of the jungle there. Hence the size of it, as it was meant to appear about the size of the Urwin in Negilahn from far away. And not meant to be seen up close. So while it's true that the Negliahn Urwin was originally going to be used in Direbo. It's unlikely that Negilahn's jungle was reworked to become Direbo (In fact screenshots and renders of Direbo from Myst V's development back this up). Plus we have Negilahn appearing finished with people in it in the original Uru Live trailer.
Todelmer - Uru Live age, reworked for Myst V. In 2004 it was still in phase 4. Which meant it was still being modeled and wired.
Noloben - This is where things get wacky. Noloben as it was going to appear in Uru was just a cove with a small house. It originally went by just "The Cove" then it was given the official name of Noloben. Parts of Noloben was used to make the one we see in Myst V. However it changed so much that a new name was given to the age. Siralehn. In fact that's what the age's files and textures are actually called. However at some point during Myst V's development, the age was renamed Noloben to make use of the reference to Uru Live and the note in the Baron's office.
Lakiahn - Was in phase 2 (The massing model stage of Cyan's design process). at the time of Uru's initial cancellation. So, while some rough modeling of it was done to give the basic layout of the age. It was far from complete and no textures or lighting had been done with it yet.
Tahgira - I'm going out on a limb and saying that despite the rumors, Tahgira was also a purely Myst V age. It never appears in the DRC list of ages. And, while a concept piece of art may have been made for it, it doesn't even look like the concept art. It's highly possible that the concept art was for Gira II. And just mislabeled Tahgira in Stephen's book. Publishers are known to make mistakes.
Oh and K'veer. - Uru Live age. Enough said.
So we have four ages, in various stages of development that were used in Myst V, and one age that was from Plasma 1.
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MOULAgain KI: 024104
Chloe Rhodes
DIRT: Uru Live