As Curl showed correctly, a pre 1904 nitro prooved gun will show as such. The 303s were marked differently than other calibers. My 1901 Jeffery 450/400 is marked "55grs Cordite 400gr bullet max'm" and my 1902 Evans 400/360 was marked "40grs Cordite 300 gr bullet max'm". What's missing is the "sceptre over NP" that came about in 1904.