Okay, I remembered that there are a couple of other possible models for that old receiver, either of which would explain the separator. It could be a 322. That is a dual-tuner standard-def receiver for two TV's. It would still require a Dish Pro LNB to be able to use the separator with a single cable from the LNB. Otherwise, it would need two cables from the legacy LNB.
The other possibility is that it is a 311k. This is a receiver that was originally manufactured as a 322, but then Dish bastardized it to be able to use it as a replacement for the 301, when they were running out of stock on 311's. On the 311k, the TV2 output is disabled, so it only acts as a single-tuner receiver for one TV. It was intended to only need one cable from the LNB to one of the satellite inputs on the back of the receiver. However, after the 311k came out, some people started complaining that the receiver was giving them problems because it did not have a satellite signal on both tuners, even though you could actually only use one of the tuners. So, Dish started installing them with separators so that both tuners would have signal, to keep the receiver happy. Dish did not keep installing this model for very long after that.