Here's a quick read on what you should have in your personal files, notify your policy holder of, and check for in your coverage on the type of value they'd use (actual cash value or replacement value).
The company that produced your property has produced a catalog of their works if you wanted to dig deep....
But luckily the company is still in operation- and are actually openly offering a way to contact for authentication and details on early works (between 10-100 Euro fee, depending on the object)
Just keep a file with any information you've gathered, multiple quality photographs, and records of authentication from the company.