shutchman,
I doubted it until I saw the chambercast,but now there is "no doubt in my military mind" that it is 6.5x70R.If you are worried about the 68.6mm length,add the 1.61mm you show for rim thickness and the result is over 70mm.I have some brass samples,but have never seen a rifle.The 6x70R Krieghoff brass,Huntington advertised should be useable after fireforming.I can't tell if the chambercast is long enough to show groove diameter.I think the bullet dia is less than .264, maybe .260-.262".Some just use .257" bullets,or you may size lead core .264" bullets to .260" or .262", depending on what slugging the bore shows.
Mike