Chris,
The round on the left is the 600JDJ from SSK. It is a 577 nitro case blown out to 620 caliber. The brass for both cases is Jamison International formerly Bell Mast. You'll have to post details and pics when available of your 600 Nitro. It sounds like a fun gun. What kind of loads are you going to run in your 600? I shoot mine a lot with cast bullets and have made a lot of different cast & jacketed mid and heavy loads for the 600's.