BTW: I can not find out what DURABOND is. Where is it sold?

Durabond is a setting product (you mix the powder with water)made by USG.
It is as hard as plaster and very difficult to sand.

Easysand is another setting product and as the name implies can be sanded.

Both products have a number after them that is the nominal setting time.
They harden be chemical reaction, not drying out like the pre-mix mud in buckets.

1940 was not commonly skim coat plaster, but 2 coat plaster.
Look at a receptacle.
If the plaster is about 3/5 inch thick with gypsum backer is it 2 coat.
A skim coat wall will be barely more than 1/2 inch thick.

If the crack is stable and not moving, mesh type drywall tape and Easysand should make a good repair.

Be sure to moisten the edges of the crack to get a good bond.

The final patch should be at least 12-16 inches wide.