Sources suggest that it's an Air Force bracelet of his Dad’s. Inscription read: Jackson Gibbs 0732545. The number is probably Jackson’s service number.
Jackson Gibbs first appeared in the episode 6.04 Heartland when a investigation led his son to Stillwater. Gibbs and his father had a tumultuous relationship while Gibbs was in high school. Gibbs left to join to the Marine Corps shortly after graduating from high school and hasn't been back in more than 30 years.
This is also the first time Gibbs and his father have spoken since Shannon and Kelly's funeral in March 1991. Yet Jackson has kept Jethro's favorite car (a 1970's Challenger R/T) in excellent shape for all this time and cheerfully returns it to Jethro when the opportunity arises.
Learn more about NCIS by checking these 11 fun facts from Foxnews.com.