Panel 15 was a product that was made as part of Weyerhaeuser's Architectural Panels Division. In 1992 that division of the company became a separate company called Citadel Architectural Products (who still makes Panel 15 as one of their products). I'd recommend contacting them, as information on the products would have transferred to them in 1992. Their website is:
Citadel Phone Number: 1-800-446-8828.