Dear Mr. Fraser,
Thank you for posting your request on History Hub!
Service numbers were stamped on World War II military ID tags and ranged between six to eight characters, depending on the service branch. World War II-era ID tags also included the name, immunizations, blood type, next of kin, and home address.
For more information, please review an article from the Department of Defense on the history of military dog tags.
We hope this is helpful.