It is worth remembering that it it the author who holds the copyright, it this case the man who drawed the maps.
He has made an agreement with the publusher of the atlas, like we do with our pictures, and allowed the publisher to distribute the maps.
Anyway... When a publishing company, does not exist anymore, it only influences the relations between the author and the publisher, ( he might not get paid?).
But he still owns the copyright to the work.
And that does expire 70 years after he is dead.
So thats the kind of atlases you should look for, old ones, where the authors have died long ago.