It's a dual active filter covering the audio range. It has two identical sections which can be set for high-pass or low-pass. By cascading a high-pass and low-pass section, you can get a band-pass effect.

Probably Krohn-Hite is the best known maker of this category of instrument.

Two general observations about the 308A, which probably apply to the SKL make in general, is that:

1. Construction quality and component selection are very good, as good or better than HP. Construction resembles Mil-spec equipment, for example with paint marks on each solder joint showing that it was individually inspected.
2. The unit was not at all well designed to be serviced. Each channel is on a separate sub-chassis, and the underside of this is not accessible without removing numerous soldered connections and pulling the sub-chassis.

A somewhat distinguishing feature of the 308A is that it is DC-coupled, so the low-pass response extends down to DC. This is rather unusual in tube audio equipment, though of course instruments such as oscilloscopes must be DC coupled.

Somewhat interestingly from a historic perspective, this is a hybrid tube/transistor instrument. The heater supply for the filter tubes (12AT7) is regulated DC, with the regulation being done using power transistors.

Also the corner frequency can be set as low as 0.2 Hz, which is well below the audio range.