In use since 1701?
Blazon: Gules a demi-dragon rampant or, on a chief of the field two keys in saltire, argent and or, surmounted by an umbraculum of the last, all bound in cord azure
In 1701, Gregorio Boncompagni’s marriage to Ippolita Ludovisi allowed him to claim the title Prince of Piombino. It had previously been held by Ippolita’s eldest brother Giovanni and then passed to her elder sister Olimpia when he died in 1699. Olimpia died a year later, and the title passed to her sister, and then by right of marriage, her husband. The Boncompagni-Ludovisi family would retain the principality until the French claimed it in the Battle of Marengo in 1800.
The arms in the chief are those of the Gonfalonier of the Church, though it is not clear when they became associated with the principality. It may have been during the brief window of 1501-1503 when Cesare Borgia briefly controlled the area during his tenure as Gonfalonier.