A quick look at Ford's Vehicle Locator shows (17) 2020 F-600 Chassis Cabs within the closest 300 dealerships, furthest being 180 miles away... 1 Retail Order, 2 Stock & 14 Fleet. Oldest unit manufactured 50 days ago. Some fleet units may have been ordered for specific fleet accounts but not to be delivered until a future date. There is no F-650 inventory showing in the same search area. OTD (Order-to-Delivery) for bodies, etc. to be installed can be a month or longer plus installation time. Inventory, regardless of order type, will show in Dealer's inventory until reported to Ford as sold and delivered. There's little reason for a dealership to advertise F-600 unless they're a large dealership and have extra inventory or inventory that was ordered for a customer that cancelled for some reason. Dealers cannot cancel inventory orders once they're scheduled.
Hope this helps!