1) The best preserved Roman military diploma to survive from the ancient world: These highly prized bronze legal documents were issued to retiring non-citizen soldiers upon successful completion of their minimum required service: 25 years in the army or 26 years in the navy..

2) Military diplomas testified that the veteran had been honourably discharged and that he and his children were granted full Roman citizenship with all its legal benefits. Diplomas were notarised copies of decrees lodged in the military archives at Rome recording every veteran..

3) This incredibly preserved diploma was found during dredging of the River Sava near Slavonski Brod, Croatia in 1997. It had been issued to a veteran of the Roman navy based at Misenum (classis Misenensis) during the reign of the Emperor Vespasian in 71 AD.

4) The diploma was witnessed on 9th February 71 AD and by the authority of the emperor grants the naval centurion Liccaius, son of Birsus, from Marsunia, honourable discharge from the Roman navy, the right to marry and full Roman citizenship for himself and his descendants.

5) Liccaius from Marsunia in Pannonia (modern Croatia), had reached the important rank of centurion which operated much as it did in the land army - Liccaius was the commander of a century, in this case the crew of a ship, overseeing all their combat, training and punishments.

6) In an utterly remarkable survival, the diploma even retains a compartment with removable cover, protecting the wax seals of those who witnessed the document - wax impressions made with intaglio rings of five of the original seven witnesses are still attached to the diploma!

7) On the bronze plate around the wax are the names of the seven witnesses that gave their seal to Liccaius' discharge, named among them are a Gaius Marcius Nobilis of Emona (modern Ljubljana) and a Lucius Mineius Iucundus from Aquileia.

8) The diploma is also historically important as the only contemporary confirmation of "Marsunia" as the ancient name for Brod, Croatia. Liccaius evidently returned to his hometown in his retirement but for reasons unknown his prized diploma was soon lost in the Sava river.

The Roman military diploma of Liccaius can be visited at the Brod Regional Museum: https://muzejbp.hr/prica-o-likaju/ 

A newly created 3D-model of the diploma can also be explored here: https://sketchfab.com/3d-models/roman-military-diploma-from-slavonski-brod-08ecca1159b64b5fa84396526de8b6ef

Thank you to @zlikovski035 for their help in creating this thread.