Not sure about actual numbers but I have an old 73 volvo and the factory manual had a section about towing. It's towing capacity was also about 1000lbs with front discs and rear drums. If it was equipped with rear discs the capacity went up to 1500lbs and if it had trailer brakes it went up to 2000lbs.
So if that logic applies here it would seem that a large part of the towing is what the truck can safely stop.
My truck came with an upgraded bumper that has a connection for a trailer with a braking system and they told me they were towing more than half a ton with it. Didn't get exact numbers and that's just talk from the previous owner.