no doubt Nowotny was a best gunmaker, gun and rifle I remember were allways top

the caliber is probably 11,6x82R 450 Express what you may know as 450 BPE 3,25"
possible the chamber is for the "Deutsche Form" or german model case what can mean you need a different sizing die because the today common 450 NE case is a little bit over case measurments. 450 NE brass can be used when resized in a custum made die set without problems but a chamber cast will tell more.
another possiblity is your DR is chambered for the 11,6x60R 450 or the 11,6x65R 450 simply the 450BPE case cut back to 60 or 65 mm.