You can find the serial number of your Bugaboo stroller by checking the middle bar of the chassis, underneath the seat. Your Bugaboo Donkey has a unique serial number on the chassis and a different serial number on the seat frame. The base numbers (chassis and seat frame in the Mono combination) are joined by the Tag number starting with "X". You will find this number on the chassis itself and in the documentation that comes with your Bugaboo Donkey. These 3 numbers are linked, and all 3 can identify your Donkey. A separate number is assigned to the second seat frame if you have a Donkey Duo or Twin extension set. Whenever you need to contact us for queries concerning your stroller, we advise you to locate the serial number or the Tag number as this will be helpful to identify your stroller and assist you efficiently.