These are usually uploaded alongside the .rar files by the original poster. Check the same source (forum, Usenet group, or cloud drive) where you found the RAR.

Use MultiPar or QuickPar to process these files. 🔑 2. Password or "Proof" Paper

Knowing the (e.g., a specific forum or file-sharing site) would help me identify if there is a known "paper" or password associated with that specific naming convention.