This add-on is to convert one set of straps to have curved ends. Curved ends are particularly useful to prevent the straps from rubbing the case of the watch. Some watches require curved ends such as F.P. Journe, JLC (Master Control), Ming etc. due to the spring bar holes being located close to the watch. If you are unsure whether your watch requires curved ends, please contact us. We offer curved quick-release spring bars in the following sizes: 17mm 18mm 19mm 20mm 21mm 22mm Please note that this add-on is for made-to-order straps ONLY and please allow an extra week for crafting curved-end straps. NOTE: Curved ends do not mean the strap sits flush with the case. This would require a special proprietary insert that matches the case of the watch, which at this time, we are not able to produce. For watches with long lugs, we now offer a curved end option with straight spring bars to reduce the gap between the watch