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 numbering 

 Referring to Karl Gustav as Charles X Gustav is a modern invention. The Swedish kings Erik XIV (1560-68) and Charles IX (1604-1611) took their numbers after studying a highly fictitious History of Sweden. He was actually Charles IV Gustav.

This is a bit cryptic; the phrase ""a modern invention"" would normally mean that he was not so known in his own time, but the next sentence suggests that he was.  —  "