I have a similar 5200. 4 port with a 2 port power supply. I bought it without the box. I didn't suspect it was different until I got home and the second joystick didn't work. I did some trouble shooting and took of the plastic from cover, and Boom! there were four joystick ports. I don't think the guy that sold it to me knew. He listed it as a 2 port, and that's why I bought it, so I won't have to use the power, video box thing, and I wanted to get the 2600 converter. I still never bought a 2600 adapter to test with it though.
I posted about it when I got it, a few years ago. Its cool to hear that someone else has stumbled onto a similar system. I think its funny that the previous owner had no idea it was a 4 port.