No example of an original Roman legionary standard or signum, is known to have survived.
The legionary standards are known to have each been topped by a eagle, but beyond this fact nothing much else is known. The popular conception of a Roman standard sporting the SPQR as a mandate from the Senate and the People of Rome has been much coloured by the epic Hollywood films such as 'The Fall of the Roman Empire' or its more-recent remake 'Gladiator'. This popular image is very likely wrong, however.
The main evidence we have to go on are coin issues, especially those of the late Roman Republic, many of which encorporate military standards within their designs.
Since the time of the great Roman general Gaius Marius, the mentor of Julius Caesar, at the turn of the First century BC, each legion was issued with a single eagle standard. As there were only around forty Roman legions in all, it stands to reason that there would have been only forty standards in total, all of which, unfortunately, have been lost in antiquity.
They were venerated as holy objects and were placed in a shrine called the sacellum at the very centre of the legionary encampment. The sacellum also housed the regimental treasury, the aerarium, so to steal from these funds was considered an act of sacrilage. Each time the standards were moved or planted in the ground was accompanied by much ceremony and, according to Vegetius, specially orchestrated trumpet fanfares.
Any legion losing its eagle standard would be considered a total disgrace, and for this reason they were jealously guarded by the soldiery, each man willing to risk his own life for the sake of his legion's eagle standard. There is a well-known story narrated by Julius Caesar in his Gallic Wars which occurred during his First Expedition to Britain in 55BC where the standard-bearer of the Tenth Legion, seeing the Roman invasion force floundering on the shore, leaped overboard onto the Kent beaches calling out to his fellow soldiers not to let their eagle fall into enemy hands, the legion were duly obliged to follow this brave fellow into battle and consequently the British forces were beaten.