Basically, what Loudthud said. During the transition period from Tube to SS rectifiers, Sunn continued to use up old stock, namely, chassis already built to have the two rectifier sockets, and simply installed an SS module into one socket and left the other unwired. I've seen a few examples of this, my 1200s has this same configuration. In fact, when I first bought it, the previous owner had popped a 5AR4 into the unused socket, not realizing it was never wired up anyways. With that said, if you have a model that has a socket that wasn't wired up, you should NOT simply be using one 5AR4 in the other socket. You need to replace it with an appropriate rectifier module.