Well, don't let me started with "šesták" ("sixer")...

One gulden had 60 kreuzer. So one sestak was a 1/10 of a gulden (=six kreuzer). Now in 1857 a decadic system kicked in and a gulden became 100 kreuzer. So a sestak was still 1/10 aka 10 kreuzer. Later on as you described a devalvation came in and it actualy became 20 kreuzer. In present times it means 20 hallers which is a fifth a of Czech Crown (CZK)