Thatcher Park is listed on NELSAP, but from contacts with park officers themselves and follow up conversations with their boss plus conversation with Ed Bunkoff a man whose info comes from his original observations, this site concludes that there was never a ski area in Thatcher Park.
Note: Ed Bunkoff ran Snowy Acres in Cobleskill, started 1938-39 season. He was a guy who walked up to ski down all over. He was also a 10th Mountain Infantry man and a reliable source.