I recall seeing the name Souper, in one of my case forming books, but can't recall details without finding the book!  Oh, it jumped out at me, thickest book I have!  Book says .25-308 for the Souper.  All I see  in 25 cal wildcats that approximate the 6mm's headsize is 30-40 and .303 wildcats.  But I'll bet somewhere along the way someone made a .25 Swift or maybe a .25 Lee!