The theory is very simple. The closed loop gain of Q911 and Q912 is set by the ratio of R86/R85, or a gain of 10. Adding an inductor into the path in the feedback loop increases the effective impedance at higher frequencies, adding "peaking" to the circuit, thus making the closed loop gain higher. I think the poor frequency response is in the transformer as well as the transistors hfe falling to under 25 typically at 30 MHz. Also, the open loop gain is also increased by bypassing the emitter resistors for higher frequencies.

The transformer is a 2 to 1 step down, (it looks like a trifilar winding) so the theoretical gain of the stage is 5.

Howard