One of the Swedish Experimental cartridges. It was not shown in any sporting catalogues until 1928, which might mean that it started out as a military experimental round. The DWM case number 431L is listed as: Schweden. Kal. 6.5mm Versuch (6.7×61 .o). The small “o” is likely “ohne” which means rimless. There was a discussion regarding the 6.5×61 and the .257 Hyper Express in the IAA Forum.