Erik Ringsson

Erik Ringsson was a Swedish king and the son of Ring, according to Adam of Bremen.
Referring to Erik Ringsson as King Eric IV or V are later inventions, counting backwards from Eric XIV (1560–68). He and his brother Charles IX (1604–1611) adopted numerals according to a fictitious history of Sweden. The amount of Swedish monarchs named Eric before Eric XIV (at least seven) is unknown, going back into prehistory, and none of them used numerals. It would be speculative to try to affix a mathematically accurate one to this king.