Here's the "T-filter" standing alone, top. Altec used it to provide supplemental HF compensation after some stock crossovers.
As you see in the M19 crossover, R2 may be varied to adjust the amount of compensation.
Probably a dumb question. Since the R2 resistor in the M19 crossover is variable why wasn't the the 8 ohm side of the Lpad used to replace the preceeding R1 resistor like in conventional Lpad use?