In 1981 one manuscript in the Sana'a cache was identified as a palimpsest: a parchment scraped clean and written over, with an earlier text still faintly present underneath. Recovering the lower layer needed ultraviolet imaging and later X-ray fluorescence, and the results were published only from the 2000s onward.
What the lower text shows is why the shelfmark DAM 01-27.1 is known well outside Yemen. It preserves a Quranic text that differs from the standard one in word order, in the presence and absence of phrases, and occasionally in wording, which makes it the only substantial witness anybody has to a textual tradition beside the Uthmanic one. It is a witness to variation rather than a rival scripture, and the scholarship that reads it is careful about the difference.
Two things complicate the custody. A small number of folios from the same manuscript had left Yemen before or during the conservation years and are in private hands and other collections, including one leaf at Stanford, which is the folio that was radiocarbon dated. And the condition of the material still held in Sana'a has been a concern since the Yemeni civil war began, for the ordinary reason that a conservation store needs power, climate control and staff to remain a conservation store. What the atlas can state is what is known to be held, where it is, and where the gaps in that knowledge begin.