Bark, I cleaned up your link so it will open.

http://s1135.photobucket.com/albums/m626/FCutten/?action=view&current=taillight.jpg

Those type of trailer hitches were common 60 years ago. I don't know if the type pictured was commercially built or not. Most of them were home made or a welding shop custom built them. This one looks like it was designed by someone who made good measurements, but the weld doesn't look like it was done by someone who welded the same thing all day long. The hitches weren't very strong, and the pickups weren't stable enough to pull a very heavy trailer. Most trailers were single axle. Tandems really didn't gain popularity until the 60's when the pickups had more power and more stable suspensions. The rear bumper is different than the front. It is shorter and has less curvature to it.