Such a reconstructed Vorlage may be called a retroversion, and it invariably is made with some amount of uncertainty.
Nevertheless, the Vorlage may still be reconstructed in some parts at such a level of confidence that the translation and its retroversion can be used as a witness for the purposes of textual criticism.
For example, scholars use the reconstructed Vorlage of the Greek Septuagint translation of the Hebrew Bible at parts to correct the Hebrew Masoretic version when trying to determine oldest version of the Hebrew Bible that they can infer.
[4] As another example, the Coptic fragments of Plato's Republic included among the Nag Hammadi library were used to help attest to the original Greek text he wrote.
[5] For the bulk of the Gospel of Thomas, the Vorlage exists only as a retroversion of the Coptic translation, as no other witness to the original Greek text for most of the sayings recorded therein is known.