I started by washing away 150 years of crud.
There are some interesting spots of some sort of reddish paint/dye perhaps ground coat?
and the edges when scraped clean show some of the original colour.
Pity I can't resurrect the original finsh. But it has completely deteriorated. This is how the varnish looks now, and look at the glorious wood underneath.
Here's how some of the inside back cleaned up. The plane is just for scale, about life size here. I washed the crud off with water and a plastic sponge, the lovely old patina remains.
