I found a #6 shellholder on my bench- it is stamped " 6 " but it matches nothing on the charts, the lip that holds the rim is 0.025" larger than a shell holder designed for .223 and the .223 is a very loose fit as the lips are 0.370" rather than 0.345" (quick rough measurement).
It does not match anything on this shellholder chart
www.reloadbench.com/gloss/rshellx.html either.
Any idea of what it is for?