61bhs,
This cartridge is typically known as 8x57JR(or IR). The 7.8 and 57 are the bore( not groove or bullet) diameter and case length and are placed by the proof house.The same numbers will be found on rifles chambered for 8x57R/360.If the cartridges you backordered will not chamber, this is likely the reason.S&B 8x57JR ammo has recently been avaliable and you might find a box "in stock" somewhere. I suggest you try J&G Sales,Huntington Die Specialities, or Buffalo Arms.Some local dealers will have some also, so that means a "yellow page search" and phone calls. Components are avaliable and it is easy to load for.
Mike