80m phase shift SSB transceiver, How does it detect USB/LSB?

Hi this is a 80m SSB transceiver based on the phasing method ../imgqa/eboard/Antenna/rf-1pu3cqedpvi.jpg

I consider the phasing audio section. In the transmit mode, 180 degrees AF phase signals are provided by the BJT terminals, then somehow shifted by these RC to produce a 90 degrees phase difference.
But on the receive he seems to just connect the two phased AF signals together. So how does detection of USB or LSB occur based on the RL3 switching?

I think it is DSB so no switching is needed.

Brian.