You have two problems.....
The first problem is that the Transfer Relay Delay (transfer switch) that you found is
NOT the switch that you were looking for. This switch is actually part of your Intellitec Electronic Climate Control system. The TRD is the transfer switch that hooks your front air conditioner to the 20amp output of your generator to allow both air units to operate reliably and simultaneously on generator power. The transfger switch is closed by the 20amp feed when the generator is running.
See this Manual. page 9 The MAIN transfer switch is the one labeled "Change Over" at the top of that diagram.
The second problem is that the MAIN transfer switch (the one you
ARE looking for) is indeed mounted on the back side of the main breaker panel. It is actually inside the panel as an integral part of the box. If you take the front cover off the circuit breakers, you will see the Black and White wires coming through a hole in the back of the panel to the main breaker. They are coming from the transfer switch.