Ah yup, looks like a newer (maybe or not nicer) version of the Eico 322 I have. Audio modulation, changes the plate voltage a little thus modulating the RF output slightly. That's what the transformer is doing, strapped on there.
The RF output level is high impedance (perhaps 1kohm at the maximum amplitude setting), not particularly powerful, and pretty high in distortion (which might be seen as an advantage or a hindrance). My 322 typically shows 10-20 nice strong harmonics on the spectrum, of varying amplitudes as the frequency and band are adjusted.
As an instrument, it's pretty pitiful. A low impedance buffer, transformer, or other matching circuit, would be highly valuable for improving power output, and generally making it useful among 50 ohm systems. Not much can really be done about the distortion, either for reducing or eliminating it, or for trying to make it more consistent (the harmonics might be useful if they were a known and stable level for all ranges). The harmonics vary because of the class C oscillator (= distortion generator), the varying capacitance (shunts the harmonics differently over frequency), and resonant modes of the coils (which are especially important on the higher ranges, where peaks and dips occur).
Tim