: This seems to indicate a sequence or an identifier number. In the context of multi-part archives, it could signify that this is the first file in a series.
These parts typically follow a strict sequence, such as filename.part01.rar through filename.partXX.rar . 081424-001-carib-4k60fps.part17.rar
You might not get 100% of the data, but in the world of digital preservation, 99% is often a victory worth celebrating. : This seems to indicate a sequence or an identifier number