Sylt is also a funny name. It means jam (preserve) in Swedish. I think it is clear who the rightful owner must be.
The Danish name for it is Sild; considering how Schleswig slowly has been Germanized I wouldn't be surpriced if Sylt is a Germanization of Sild---I don't know if it has a Frisian name. On my replica of a map of Denmark from 1585 it is called Sildia, so the Danish name at least dates that far back---Sildia is obviously a Latinization of Sild considering all the rest of the map uses Latin names.
Sylt has also been used for long though. I had a look in the
atlas of Frederik V who ruled Denmark from 1746 to 1766. The three following maps are from the 35th volume of the atlas.
The forum apparently aren't happy about showing the pictures, since it eats them, so you get the links to the pictures.
This map uses Sylt in 1649
https://images.kb.dk/present?id=fr535029
This one also uses Sylt
https://images.kb.dk/present?id=fr535011
And so do this
https://images.kb.dk/present?id=fr535001
Given how the court of Denmark spoke German in those times---I don't know when they switched to French if they ever did---my guess is that it is the German name that is used in the three images and that Sild never have been Sylt in Danish.
By the way the atlas covers the entire globe so have a look at it if you ever need maps from before 1766; there are some from the 1600s and perhaps some even older. There also is several drawings of large fortifications around the world, military equipment like cannons. In the last three volumes there also is buildings, distance tables, flags, animals, satirical printings, ships and more things.
In all there is 3535 sheets distributed over 55 volumes and it all is available from the Royal Library of Denmark through the link I gave above.
