If you are looking at this then you know what it is. It is added to a strap or bracelet of a Breitling watch allowing for a second timezone. it is 20 mm at 12:00 and 18 mm at 6:00. This one is in nearly new condition, running perfectly and keeping a great time with a fresh battery installed.
Breitling UTC Module ref. #B61072 SS/18K Blue Dial #2928