Curaçao was one of the Dutch overseas territories where a lot of postal stationery was issued in connection with regular stamp issues. I've showed you a copy of one of the Silver Jubilee stamps the other day (24 April). The portrait was used for the next set of definitives to be issued there in 1928, set in a new border. Again, these stamps were recess-printed. The most common values were also available on prestamped envelopes.
As you can probably see from the scan, these were printed in photogravure.
I think these are a lovely addition to the stamp set. And they even mirror all changes in the stamp set. So when the 7.5c stamp was surcharged 6c, pending the issue of a proper 6c stamp, the 7.5c prestamped envelope was also surcharged and later replaced with a proper 6c prestamped envelope! I may show you that one at a later stage, when I've found it again!