The Authors Guild suggests publishers relinquish their Anthropic claims in instances where rights have likely (or should have) reverted
The Guild says lack of clarity over rights reversion is the most common problem for claims due to lack of publisher records.
https://janefriedman.com/the-authors-guild-suggests-publishers-relinquish-their-anthropic-claims-in-instances-where-rights-have-likely-or-should-have-reverted/