National Association for Olmsted Parks
The National Association for Olmsted Parks (NAOP), established in 1980, is a coalition of design and preservation professionals, historic property and park managers, scholars, municipal officials, citizen activists, and representatives of numerous Olmsted organizations around the United States. Its concern is the legacy of landscape work left by Frederick Law Olmsted Sr. and the firm founded by his sons, Olmsted Brothers Landscape Architects (OBLA).
Appears to be an umbrella organization, not affiliated with a specific historic site.