Market Reef, a small uninhabited island in the Baltic Sea, between Finland and Sweden, IOTA reference EU-053, is an interesting island from a DXCC perspective.
Market Reef has been partly owned by Sweden, and partly by Finland, ever since 1809. On 1985-08-01, the border between the Finnish and Swedish part was changed to the current zig-zg line.
The Finnish (eastern) part of the island is separated from Finland by another DXCC entity, Aaland Islands.
Under the DXCC Rules, Section II, DXCC List Criteria Geographic Separation Entity rule, 2 b) ii), this means that the Finish part of Market Reef qualifies as a separate DXCC entity in its own right.
Consequently, there have been a significant number of amateur radio operations from the Finnish part of Market Reef.
On the other hand, there is no separate DXCC entity lying between the Swedish (western) part of Market Reef and Sweden. Therefore, the Swedish part of Market Reef counts as Sweden for DXCC purposes.
Consequently, there have been a limited number of amateur radio operations from the Swedish part of Market Reef, for example SI8MI and 8S9M.
ARRL: DXCC Rules