Is this like the works of HG Wells? In the US they've been in the public domain since about the 1930s (at least his early stuff from the 1890s: Time Machine, Invisible Man, War of the Worlds, Island of Dr. Moreau) while in his native UK they were under copyright until 2017 (death + 70 years).
Except REH was a Yank, so the UK only granted protection as long as the US did. And the early stories have been public domain in the US for decades.
These stories were bound by the 1909 Copyright Act, which required explicit statement in the work establishing copyright, and later required the owner to file for renewal. This wasn't done.