For a mid-20th-century audience with a passing familiarity with contemporary communist politics, it would have been much clearer than to an arbitrary person tod

For a mid-20th-century audience with a passing familiarity with contemporary communist politics, it would have been much clearer than to an arbitrary person today. Contemporary reviewers were very convinced, and also thought that the written passages from Goldstein's in-universe book were call-outs and/or parodies of specific passages in Trotsky's "The Revolution Betrayed", I don't know enough about that book to comment on parallels, but Orwell's contemporaries did not consider the Goldstein/Trotsky reference especially subtle.