Photo from Réunion des musées nationaux via A London Salmagundi.
Addendum: The object has been identified by Seattle Sailor as a "portepee":
They were originally used to help retain the sword during combat but later evolved into a strictly ornamental device denoting rank. I can't see a sword in this picture, but they were also worn on daggers, which may be the case here.Here's a photo from the German Imperia Automobile Corps illustrating portepees (and leather "frogs" on daggers:
The Prussian Automobile Corps authorized a distinctive portepee for wear with these daggers. These silver bullion strap portepees with red and black stripes are extremely rare and rarely seen.I am recurrently surprised and delighted about how I can post questions about the most arcane topics on this blog and get an authoritative answer within hours. Amazing. Thank you.