I found this http://www.brastad.nu/Pages/Restext.htm looking for the split island Hison or Hisön. This site is about a school expedition along the NO-SE border. I think the first picture on the first photo shows the border line (a marker and the cut trees like in N-America).

And a map http://www.frilufts.se/goteborg/orange/leder/bohusled/bohus9.html According to the text boundary marker 1 is on island.

There even apears to be a tripoint on the island between Norway and two Swedish municipalities.

More informartion here, but in Nordic language I am afraid. Peter and Emil might appreciate it. http://www.abc.se/~m225/litteratur/lig/lignell_gransb.html

Here is some technical info on the island.
http://www.calle.com/info.cgi?lat=58.9000&long=11.6500&name=His%f6n&cty=Norway&alt=482

Bear in mind when you search for Norwegian and Swedish places that the Nordic letter "oe" is ø in Norwegian (in Danish too), but ö in Swedish (like the German version). I hope your e-mail-programs can show the propper symbols,

Jesper