Non-restrictive relative...

Spotted this in the newspaper-article-intro to the 1940 film Phantom of Chinatown:

The scientific world is anxiously awaiting the report that Dr. John Benton, of Southern University, will make on his recently returned expedition into Mongolia. It is rumored that Dr. Benton discovered the long lost solution to the Temple of Eternal Fire, that has been the goal of Archeologists for centuries. If true it should prove of tremendous financial importance to China. Wow.

A relative non-restrictive that! You don't see that every day...