There are 4 different TX filters, controlled by 3 signal pins. In the firmware source code, there is are comments:
21 MHz and up: the default filter is with 35 MHz cut-off, (all of the TX_LPF_x pins are low)
14 -21 MHz: thrown the KT1 relay on, the 30 MHz LPF is bypassed and the 14-18 MHz LPF is allowd to go through pin (TX_LPF_A is high, the rest low)
Looking at the v6 schematic, I don't see a KT1 relay. It looks like TX_LPF_A triggers KA1. Also, as I trace the schematic, it appears that one filter (the bottom one on the schematic, I assume the 35 Mhz LPF) is always in the circuit. As I missing something?