The passage, from the work of Marlin Barleti from 1443, states: “Nam rubea vexilla nigris et bicipitibus distincta aquilis” meaning “The red flag is distinguished by black and double-headed eagles”. This shows that the idea of a proto-Albanian state with its own flag existed already in the 1440s under Skanderbeg.

Source

https://archive.org/details/historiadeuitaet00barl/page/n2/mode/1up