Raimey,

The gun diagrammed in the bottom picture is NOT associated with the 8x58 Sauer and Sohn Express casting photo you included. They are two different guns.

The diagrammed gun is a 8x57JR. I took a casting and then used a dial caliper to precisely measure each location of the casting to make 100 percent sure it was truely the 8x57 and not some sort of very closely resembled extinct cousin. Of course, the 8x57 has a neck on it and looks more like a 7.62x39 or a 7.62x51 than does that odd extinct 8x58.