Large spline Maillard freewheel remover. Note the original Maillard tool will crush in a bench vise. It had a small Atom Spline on the inner side so we put an Atom freewheel remover in the Maillard remover to keep from destroying it. There were other brands made that were more durable. Bicycle Research made theirs with a large hex on the end for use in a bench vice or a with a big wrench.
Or take it to an old school bike shop.
Looking on ebay $25+shipping.