I looked this gun over as well, it is a nice gun, bores are very good. What was interesting is it was never proved for cordite, the barrels are engraved listing a cordite charge and the 480gr bullet. The proof marks simply state 450 ex.

It's a 1901 gun and is the second Rigby 450 I've seen from that year that was marked as a cordite gun on the barrels but not proved for it. The first was a hammer gun with the exact same cordite engraving on the barrels. Guns from that year would have carried cordite proofs if proven with a cordite load. The "NP" nitro mark didn't come out until 1904 but the flats would have been marked:

75grs cordite, 480 grs bullet MAXm, if proven with cordite

My 1902 Evans 400/360 and the 400/350 1901 Rising Bite that Westley sold at DSC are so marked.

At that weight it was probably built as a nitro gun but I'll let someone else risk it.

If it had been proved for Cordite that gun would never have seen Dallas, it would have been in my safe three weeks ago