The significance of a non-factory serial number etched on a bike's frame can vary depending on the circumstances. Here are a few possible explanations:
Theft prevention: Some bicycle owners choose to etch their own unique identification number onto their bikes as a theft deterrent. By doing so, they make it more difficult for thieves to sell or pawn the bike, as the altered serial number raises suspicion.
Personal identification: The owner may have engraved their personal identification number (e.g., driver's license number) or another unique identifier to help identify the bike as their own. This can be helpful in case of theft or when proving ownership.
Customization or modification: If the bike has undergone modifications or customization, the owner may have added a unique identifier to differentiate it from other similar models or to showcase personalization.
Law enforcement or registration: In some regions or countries, bikes are required to be registered with local authorities. The etched number could be part of an official registration or identification system used by law enforcement agencies.
It's important to note that the specific significance of the number can only be determined by the owner of the bike or the person who etched it. If you're curious, it's best to ask your friend directly about the origin or purpose of the etched number on his bike.