Apple tree with rotting trunk.

[FB.]

What rootstocks would they have been using in 1940, FB?

Quite a few possibilities.

Vigorous to very vigorous rootstocks could have included: M1, M2, M3, M4, M16, seedling and possibly some of the other M1-M16 series.
The MM101-116 series and M25 also existed so could have been used in theory, although they were not officially released for another decade if at all.

[yummersetter]

We have a Bramley that had a chunk taken out of the trunk by a 1960s sheep that looks similar but a few feet up. Woodpeckers made a hole through higher up where a branch had been removed and eventually the two holes joined inside the tree. Blue Tits nested in the top hole and the fledglings would helter-skelter down tthrough the tree and fly off.

I did try filling the tree tunnel years ago with expanded polystyrene foam to keep the water out. It now looks like an intriguingly rare fungus.

[veggiechicken]

The good news is the apple tree appears to have withstood its battering My neighbour has cut back all his tree's overhanging top growth and has hopes of saving his tree by supporting it on props. Photos will follow!

[veggiechicken]

Photos! The good news is that my tree will have more light now - it lives to fight another day