Good point on stability Wayward. I hadn't thought about that. The rest of the box is very solid, but no sense in doing anything that would destabilize it as it ages over time. One piece would mean less nailing too. Is there something like the fake wood paneling used in mobile homes, but made of real wood? Thin stuff made to look like multiple boards, but in a sheet? The pieces that are on there now probably add up to 1/4" but are maybe 1/16" to 1/8" each. I'm going to Home Depot on my lunch hour to pick up a couple of things and I'll do some looking to see if I can find a suitable replacement. I'll try some local lumber places too.
I would have liked to use antique materials, but I don't know if it is going to be possible on this. But I think that I will keep it as non-invasive as possible, with the idea that I might be able to find a sheet of something period correct that could be put in at a later date.
I agree, the backing should go, as much as I hate to remove something original, there is a point when original condition ceases to be a plus and becomes a minus. Looks like it's also torn up from the bottom edge, like critters were going in after crumbs, since they didn't have a key to open the front!
On the split, I could use a C-clamp and some wood glue quite easily to mend that crack and keep it from getting worse.
I think the only way to date this is going to be by the screens, hinges, swing latch and lock. I'll try to get some better photos of these tonight.