A view of the rear of the chamber should tell you if it is chambered for a necked .577 case.

Since the ladder goes to over 700yards, indicating a long range capability, I suggest a 480gr. to 500gr. bullet - thus, perhaps a .577/450. The .450 #2 and .450 Express light weight bullets to 350gr. would not have such - narrow spacing on the sights to 700yards, I think, - just an offhand thought.