Here is a Curaçao one from
ebay with the somewhat different design and view of St. Annabaai, the natural harbour of Willemstad on Curaçao.

The Dutch Antilles, originally, were called Curaçao after the main island, or Curaçao and Dependencies. The name was changed to Nederlandse Antillen.
As you can see, this design of Curaçao has the lower case 'f'. My guess is that when the name changed and the design was changed, the lower case 'f' was retained. Already in 1948/1949 (the time when the name changed) the upper case 'F' was introduced.
All pictures I can find have the upper case. The one with the lower case 'f' may have been very shortlived.