Sounds like someone has rebored the chamber from a 9.3 Case of some sort to a
375 case - like the 375 2 1/2" OR they have a 375 bore and a 360 Case.

They have then stamped the new load on the flats - 65 grains imr 4831 x 285 grains

Cordite 40 - 300 MAX is original proof load - sounds like one of the 400 / 360 cases to me.

The type and age of the gun may be of help in working this out.

If you can describe the gun and / or post a photo, this would be good.

Hope this helps

500 Nitro