If you are looking for a specific you've encountered, could you describe where you found it or its subject matter ? This will help narrow down its origin.

It appears as a mortgage-backed security identifier (e.g., FHLG 30YR 3.5% #G60393) in various Fidelity portfolio holdings .

It is a reference code on Alamy for stock imagery, specifically of soccer player Robin van Persie during a Manchester United vs. Reading match at Old Trafford.