I've started cleaning up the metal parts. The wooden blocks on box ends, where the rope handles go through, don't come off. They are held in place by two screws (easily undone) and two other fasteners that don't have any visible way to undo them. Possibly some form of rivet?
The hinges are seriously heavy duty, and were actually welded together, not simply roll formed. They will never fall apart.
The metal strap of the lock ie also held by a combination of screws and rivete, so it to will be staying put.
I have spoken to a leatherwork business, and they are custom making a replacement leather strip for around the edge of box lid. It secures the canvas in place. Vegetable tanning was used for industrial leather usage around the time the box was made, so that is how I have requested the strip. It will be very dark brown or black in color, and 3mm thick. After measuring the current leather thickness, my leather guy could work out how thick the original would have been. He feels it may have shrunk by ahout 0.5mm over the last 70yrs. All going well, it should be ready to despatch to me by early next week.