The Reliant was a ship he served on as an Ensign according to a deleted scene in 'Measure of a Man'. Said scene was included in the extended cut of the episode on the Blu-ray release.
https://memory-alpha.fandom.com/wiki/USS_Reliant_(24th_century)
So some time between 2327 (when he graduated from the Acadamy) and 2333 (the earliest we know he served on the Stargazer)
Thanks. So Picard was an ensign at the time. Hopefully if we see the Reliant, it will look more like the Stargazer than that ship in the certificate. Unless they've decided to 'visually reboot' the Stargazer as as well.