The black or white leather strap is interchangeable one with the other. The securing mechanism is a small round screw which is undone with a 5 centime piece or similar. As you can see from the photo, there is a hole in the strap where the watch is fitted. 14mm width.
Mondaine strap for the Aura.
Petite, yet perfectly formed, the Aura watch offers all we love about Mondaine, but in miniature size. With the crown uniquely gracing the top of the case, the Aura is also distinctive for its easily interchangeable straps. You can gain a whole new look by changing the strap from leather to metal in a flash. The strap width is 14mm. If you are unclear on the correct watch strap see this page.
Authentic Mondaine Swiss Railway Watch Strap with Mondaine stamped on the inside of the leather strap and embossed on the buckle.