Bowen 475, it would be an 8 bore in the chambers as far as ammunition is concerned but the number 9 referes to the internal diameter at 9" from the breech face. This will indicate its proof status and the correct measurments at time of proof so as to show if any metal has been skimmed out. It is not unusual to seen 12 bore marked as 13 bore or even 14 or 15 bore. If greater than .813 at this point it would be deemed as out of proof here in Britain and unlawful to sell until re-proofed. This only applies to British proof laws. Have it checked for measurement by a good gunsmith. If in proof I would happily use it with the intended loads. To re-proof you would need to send it to a Country that has a Government Proof House; preferably the one in Birmingham or London. You will find that it will throw the best patterns with black powder in any case. I would not use nitro powder until shown to be safe with it. Hope that helps a little. Lagopus.....
