Old Tibetan writing has a feature that later orthography abandoned: a form of the vowel sign i written the other way round, mirroring the ordinary shape. It is known as the reversed gi gu, and its presence in a text is one of the clearest signals that the text or its exemplar is early.
What made this observable in quantity was the recovery of a very large body of Old Tibetan documents from a sealed library cave at the Mogao site near Dunhuang, on the eastern edge of the Taklamakan, opened at the turn of the twentieth century and dispersed to collections in Europe and Asia. The cave held manuscripts in many languages, and among them a substantial Tibetan corpus from the period of Tibetan control of the region: contracts, letters, administrative documents, divination texts and Buddhist works.
The importance of that corpus is that it is not a canonical tradition copied and recopied. It is ordinary written Tibetan from a datable period, and it shows the orthography as it was actually used before the great standardisations of vocabulary and spelling. The reversed gi gu, the second suffix da, particular particle spellings and a set of other archaisms all appear there.
What later scribes did with these features varied. Some reproduced what was in front of them and some normalised it, so an archaism in a manuscript is evidence about its exemplar as much as about its date.