Metasploit's payload UUID is a unique identifier used in Metasploit's HTTP and TCP payloads. URLs used in payloads will encode the payload UUID in BASE64, with metadata (platform, architecture, timestamp) and checksum.

We can use this to check (metatool.py) if a URL was generated by Metasploit:

Didier Stevens
Microsoft MVP Consumer Security
blog.DidierStevens.com DidierStevensLabs.com