I don't have any docs. It's basically:

600 ohm OSC BNC input -> 600 ohm step attenuator 
   -> tapped secondary output transformer -> output terminals

If you don't have a 600ohm source, then attenuation will be off.

input terminals ->
   tapped primary input transformer -> optional 600ohm load 
     -> VM output to hi-Z voltmeter

The >5Kc switch switches between two entirely separate pairs of transformers in the unit. The CT terminals are the center tap of the windings, and can be grounded to insure a balanced signal w.r.t. that ground, and also to intercept any common-mode current coupled through the transformer inter-winding capacitance.

The input-side load is switched off in the BRG (Bridged not terminated) setting. I'm not sure what transformer tap is used in that case.

I believe that in the CAL switch setting, the external binding posts are disconnected, and internally shunted together, allowing you to determine the transformer loss.

I gather that in its typical application for telco line-loss measurements, you would set the generator and VM for a zero-db reading with the attenuator at some starting position, say -20db in CAL, then switch to meas, and then adjust the attenuator for the same reading. I suppose you could read the attenuation directory off the meter too, but that is perhaps less precise.

The main purpose that I have used this unit for is to generating <1mv balanced output signals in the AF range. This gets around problems with the inevitable 60hz ground current in line-powered generators. The hum at the output of the 353 is down in the microvolt range. There is some hum pickup in the transformers from fluorescent ballasts, etc. If you have some residual hum, try rotating the 353 various ways to see if this makes any difference. In this mode, I don't use the input side of the unit at all.

As I recall, when using the 353, you want a primary (i.e. OSC) side earth ground, e.g. by strapping your oscillator output to earth ground on the back. This provides a return path for the ground current intercepted by the transformer's internal shields.