Some watches require a curved spring bar to hold the watch strap tightly against a curved watch head.
As with straight spring bars to determine the correct size simply measure the distance between the lugs on the watch head. This will give you the correct spring bar length.
The thickenss of the spring bar is most likely to be 1.8mm but some watch straps or watch bracelets require a thinner spring bar. In this case choose the 1.5mm diameter spring bars. If in doubt choose the 1.5mm as it will not matter if the spring bar is slightly thinner than necessary.