Broken Serpent Ring

Jewelry is common across the world and takes myriad forms, which is why one particular ring has become such an effective symbol: it hides in ubiquity. The 'Broken Serpent' ring is little more than a band of metal fashioned like a serpent and worn as a wrap around ring, with the fangs on the outward side of the finger from the tail. The design is innocuous, and indeed a wide variety of people who aren't part of the resistance movement within the Empire wear these rings freely.
The Broken Serpent alludes to a sect who call themselves the Ouroboros, those convinced that the Emperor is perverting the natural order with his very existence and seeks to stifle natural cycles throughout the world. The serpent eating its tail represents these cycles, so the 'broken serpent' is a nod toward a breach that must be corrected. In order to maintain obscurity, members of the order not only wear rings of a specific material (differing based on their role within the organization), but also employ challenges and passwords that change on a weekly basis, and are only uttered if someone has the ring AND has it in the proper location (which changes month by month). So far the Ouroboros has avoided extermination at the hands of the Praetorians, but has little power to do anything other than send intelligence to the Empire's enemies. A massive uprising would not be successful in uprooting the Emperor, at least not without destroying half of Bellarus. Careful ciphers are also employed, and broken circle iconography is sometimes used to mark locations or letters (though the Praetorians are dimly aware of this symbol, and it's now become more of a decoy.)
Because wrap-around rings are popular in the Empire right now, members have an easy time dissuading investigation by simply playing ignorant when someone they don't know uses a challenge word, even if it's correct and current. Banning rings in the city is wildly impractical (and would be met with scorn by goldsmithing and silversmithing guilds, who are not an insignificant presence), so for now the symbolic rings serve their purpose ably.