Caesaria, the thunderbolt was an ubiquitous decorative pattern that was found on several military items: shoulder guards of cuirasses, cheek guards of helmets, belt plates, sword scabbards, and of course shields. It was very widely used and was probably believed to bring Jupiter's protection over the wearer.
Later on during the 3rd century that protection was in the form of decorated roundels used on the sword baldrics bearing the inscription "Jupiter Optime Maximo Conserva Numerum Omnium Militantium", meaning "Jupiter Greatest and Best Protect all the soldiers of this unit" (numerum). They would also wear belts bearing the words "Utere Felix" (Always Lucky) written wirth bronze letters riveted to the leather.