You ask "What separates a Fox from a Flues?" The Flues has a section of the receiver that is sometimes thinner than it was probably designed to be. The frame splits on the Flues can usually be traced to this thin section. Possibly the pictured Fox split receivers have a slightly thinner than normal section that caused or allowed the splits. I have been told that the thin part of the Flues receiver can be measured for acceptable thickness by those who choose to shoot them with modern ammunition. I don't know how this is done or where the measurement is taken. The gun must be disassembled as I understand it.
